The giant panda is well-known for its black-and-white fur. However, there are some rare pandas with brown-and-white fur. Scientists in China have recently discovered why this happens. They studied the genetics of pandas both in the wild and in captivity to reveal the mystery of the extremely rare pandas' unusual coats.To understand what lies behind the trait, the researchers studied Qizai, a male brown panda rescued as a cub in 2009 from Foping National Nature Reserve in Hanzhong. He is currently the only brown panda in captivity. When compared under a microscope with hair samples from three black-and-white pandas, Qizai's brownish fur had fewer and smaller melanosomes, the structures responsible for fur color. What's more, the melanosomes were more likely to be irregularly shaped, the study team found.The scientists also traced Qizai's family tree. While none of Qizai's immediate family members had brown fur, they discovered that his parents and son all had one copy of the recessive trait on a gene known as Bace2, while Qizai had two copies. An individual's genes can carry recessive traits, such as blue eyes or red hair in humans, without appearing as a physical characteristic. Each parent must possess a copy of the genetic variant and pass it on in order for the trait to appear in the offspring, as is the case with Qizai.To confirm this, scientists used a gene-editing tool to change a similar gene in mice, resulting in light brown fur. This suggests that the same mutation (突变) might be responsible for the brown fur in pandas.However, it's not clear what caused the genetic mutation. The senior author Dr. Fuwen Wei said it might be linked to the specific environment of the Qinling Mountains, which has a different climate than Sichuan. The researchers believe the brown color is likely due to natural variation rather than inbreeding, which is positive news for panda conservation.4.Which of the following words best describes the research process mentioned in the passage?A Spontaneous.B Theoretical.C Rigorous.D Observational.

The giant panda is well-known for its black-and-white fur. However, there are some rare pandas with brown-and-white fur. Scientists in China have recently discovered why this happens. They studied the genetics of pandas both in the wild and in captivity to reveal the mystery of the extremely rare pandas' unusual coats. To understand what lies behind the trait, the researchers studied Qizai, a male brown panda rescued as a cub in 2009 from Foping National Nature Reserve in Hanzhong. He is currently the only brown panda in captivity. When compared under a microscope with hair samples from three black-and-white pandas, Qizai's brownish fur had fewer and smaller melanosomes, the structures responsible for fur color. What's more, the melanosomes were more likely to be irregularly shaped, the study team found. The scientists also traced Qizai's family tree. While none of Qizai's immediate family members had brown fur, they discovered that his parents and son all had one copy of the recessive trait on a gene known as Bace2, while Qizai had two copies. An individual's genes can carry recessive traits, such as blue eyes or red hair in humans, without appearing as a physical characteristic. Each parent must possess a copy of the genetic variant and pass it on in order for the trait to appear in the offspring, as is the case with Qizai. To confirm this, scientists used a gene-editing tool to change a similar gene in mice, resulting in light brown fur. This suggests that the same mutation (突变) might be responsible for the brown fur in pandas. However, it's not clear what caused the genetic mutation. The senior author Dr. Fuwen Wei said it might be linked to the specific environment of the Qinling Mountains, which has a different climate than Sichuan. The researchers believe the brown color is likely due to natural variation rather than inbreeding, which is positive news for panda conservation. 4.Which of the following words best describes the research process mentioned in the passage? A Spontaneous. B Theoretical. C Rigorous. D Observational.