Recently，a group of Chinese scientists took things a step further by using 3 D printing technology to create realistic models of human organs.In the past，it was challenging to create a single material that could print all human organs because they are very different from each other.The Chinese research team started by using hydrogel，a soft gel-like material，as the main material. Scientists use hydrogel and other materials for printing human organs.They have various hardness，such as our bones being the hardest and our brains being the softest.To copy the hardness of human organs，the researchers added metallic elements to the hydrogel and regulate their ratio.In this way，some models can be hard，like the bone，and some can be soft.However，the structure inside the human body is also unbelievably complicated，even in the same organ，different parts require different levels of hardness.To make sure the printing results are correct，the team used a specific technology called light-curing in the 3 D printing process.A special light was placed under the printing machine，causing the gel to become strong right after it was printed. ___________.They usually cannot be used for organ transplanting directly，because human cells are likely to recognize them as "enemies" and start to fight against them.These days，scientists are working on bio-printing：using 3 D printing technology to build up various types of cells and materials layer by layer.After that，the printed product will be more acceptable to grow inside the human body.However，this method is in development，and experts are still on the way.They hope it will be used in the medical field within a few years，according to CNN. These 3 D printed organ models are used in various fields，such as testing medical tools and presenting operations.Compared to live organs and animal models，they are also ethically acceptable since they are man-made and printed.To some degree，they can help researchers develop new treatments more efficiently，due to their lower relative cost，commented the Alliance of Advanced Biomedical Engineering website.（1）In the past，what was the main challenge when printing human organs？ BA．The cost of printing organs was too high to afford.B．It was hard to use one material for different organs.C．There were too many organs for researchers to print.D．The printing technology was still in the development.（2）The research team make the printing results right by C.A．adding more gel in the process of printingB．controlling the number of metallic elementsC．using light-curing technology while printingD．putting a special light over the printing machine（3）Which sentence can be put in the " A" in Paragraph 3？A．However，the hydrogel materials are not perfectB．The hydrogel materials are quite expensive to developC．Besides，the hydrogel materials have more advantagesD．The hydrogel materials have been applies in many ways（4）From the passage，we know that hydrogel is A.A．used as a main materialB．a hard material like metalC．accepted by human bodiesD．invented by Chinese experts

Recently，a group of Chinese scientists took things a step further by using 3 D printing technology to create realistic models of human organs.In the past，it was challenging to create a single material that could print all human organs because they are very different from each other.The Chinese research team started by using hydrogel，a soft gel-like material，as the main material.
    Scientists use hydrogel and other materials for printing human organs.They have various hardness，such as our bones being the hardest and our brains being the softest.To copy the hardness of human organs，the researchers added metallic elements to the hydrogel and regulate their ratio.In this way，some models can be hard，like the bone，and some can be soft.However，the structure inside the human body is also unbelievably complicated，even in the same organ，different parts require different levels of hardness.To make sure the printing results are correct，the team used a specific technology called light-curing in the 3 D printing process.A special light was placed under the printing machine，causing the gel to become strong right after it was printed.
    ___________.They usually cannot be used for organ transplanting directly，because human cells are likely to recognize them as "enemies" and start to fight against them.These days，scientists are working on bio-printing：using 3 D printing technology to build up various types of cells and materials layer by layer.After that，the printed product will be more acceptable to grow inside the human body.However，this method is in development，and experts are still on the way.They hope it will be used in the medical field within a few years，according to CNN.
    These 3 D printed organ models are used in various fields，such as testing medical tools and presenting operations.Compared to live organs and animal models，they are also ethically acceptable since they are man-made and printed.To some degree，they can help researchers develop new treatments more efficiently，due to their lower relative cost，commented the Alliance of Advanced Biomedical Engineering website.

（1）In the past，what was the main challenge when printing human organs？

B

A．The cost of printing organs was too high to afford.
B．It was hard to use one material for different organs.
C．There were too many organs for researchers to print.
D．The printing technology was still in the development.
（2）The research team make the printing results right by 

C

.
A．adding more gel in the process of printing
B．controlling the number of metallic elements
C．using light-curing technology while printing
D．putting a special light over the printing machine
（3）Which sentence can be put in the "

A

" in Paragraph 3？
A．However，the hydrogel materials are not perfect
B．The hydrogel materials are quite expensive to develop
C．Besides，the hydrogel materials have more advantages
D．The hydrogel materials have been applies in many ways
（4）From the passage，we know that hydrogel is 

A

.
A．used as a main material
B．a hard material like metal
C．accepted by human bodies
D．invented by Chinese experts