Lighthouses are towers with strong lights that helped mariners plot their position, inform them that land is near, and warn them of dangerous rocks and reefs. They are placed at prominent points on the coast and on islands, reefs and sandbars.Every lighthouse has a distinctive pattern of light known as its characteristic. There are five basic characteristics: fixed, flashing, occulting, group flashing and group occulting. A fixed signal is a steady beam. A flashing signal has periods of darkness longer than periods of light, while an occulting signal's periods of light are longer. A group-flashing light gives off two or more flashes at regular intervals, and a group-occulting signal consists of fixed light with two or more periods of darkness at regular intervals. Some lighthouses use lights of different colors as well, and today, most lighthouses are also equipped with radio beacons. The three types of apparatus used to produce the signals are the catoptric, in which metal is used to reflect the light; the dioptric, in which glass is used; and the catadioptric, in which both glass and metal are used.In the daytime, lighthouses can usually be identified by their structure alone. The most typical structure is a tower tapering at the top, but some, such as the Bastion Lighthouse on the Saint Lawrence River, are shaped like pyramids, and others, such as the Race Rock Light, look like wooden houses sitting on high platforms. Still others, such as the American Shoal Lighthouse off the Florida Coast, are skeletal towers of steel. Where lighthouses might be confused in单选题1. Which of the following is NOT mentioned in the passage as one of the functions of lighthouses?A. To help sailors determine their location.B. To warn of danger from rocks and reefs.C. To notify sailors that bad weather is approaching.

Lighthouses are towers with strong lights that helped mariners plot their position, inform them that land is near, and warn them of dangerous rocks and reefs. They are placed at prominent points on the coast and on islands, reefs and sandbars. Every lighthouse has a distinctive pattern of light known as its characteristic. There are five basic characteristics: fixed, flashing, occulting, group flashing and group occulting. A fixed signal is a steady beam. A flashing signal has periods of darkness longer than periods of light, while an occulting signal's periods of light are longer. A group-flashing light gives off two or more flashes at regular intervals, and a group-occulting signal consists of fixed light with two or more periods of darkness at regular intervals. Some lighthouses use lights of different colors as well, and today, most lighthouses are also equipped with radio beacons. The three types of apparatus used to produce the signals are the catoptric, in which metal is used to reflect the light; the dioptric, in which glass is used; and the catadioptric, in which both glass and metal are used. In the daytime, lighthouses can usually be identified by their structure alone. The most typical structure is a tower tapering at the top, but some, such as the Bastion Lighthouse on the Saint Lawrence River, are shaped like pyramids, and others, such as the Race Rock Light, look like wooden houses sitting on high platforms. Still others, such as the American Shoal Lighthouse off the Florida Coast, are skeletal towers of steel. Where lighthouses might be confused in 单选题 1. Which of the following is NOT mentioned in the passage as one of the functions of lighthouses? A. To help sailors determine their location. B. To warn of danger from rocks and reefs. C. To notify sailors that bad weather is approaching.