A、They have been replaced by the advanced satellite and radio signals. B、Captains thought them useless during their voyage. C、Th

It has been said that lighthouses, casting a glow over the dark, mysterious sea, are to America what castles are to Europe—treasured landmarks—although there are lighthouses dating back to Roman antiquity there, too. In the Western Hemisphere, remains of crude lighthouses built by Central American Mayan people date to the 13th century. Alas, history’s most famous lighthouse, constructed by the Greeks on Pharos Island in the harbor of Alexandria, Egypt, in the third century BC, is a goner. Being one of the Seven Wonders of the Ancient World and what is thought to have been mankind’s tallest structure for centuries, it was damaged over time by a series of earthquakes. Finally, it toppled for good in 1480, shortly before Christopher Columbus set out on his voyage across the Atlantic. An Egyptian sultan pulled some of the heavy stones from its rubble to build a fort on the island. The Greeks soon applied the island name, "Pharos," to lighthouses, which were a new concept at the time. This then became the root of words referring to lighthouses in many romance languages. "Faro" in Spanish, for instance, and "phare" in French. Centuries later, satellite and radio signals have rendered lighthouses obsolete for shippers and sophisticated mariners. But to captains of small boats, a lighthouse is still a valuable and welcoming sight in a storm, and a guide past treacherous rocks, reefs and shoals, just as it was when hardy keepers maintained the lights.
19. What do lighthouses mean for America?
20. What can we infer about Alas from the passage?
21. Why are lighthouses not widely used today?