Passage Two Qustions36 to 40 are based on the following passage． For centuries man dreamed of achieving vertical flight．In 400a．d．Chinese children play with a fan—like toy that spun upwards，and fell back to earth as rotation ceased．Leonardo da Venci conceived the first mechanical apparatus．Called a：Helix”, which could carry a man straight up but this was only a design and never tested. The ancient dream was finally realized in 1940 when a Russian immigrant, an aeronautical engineer, piloted a strange—looking craft of steel tubing with a rotating fan on top．It rose awkwardly and vertically into the air from a standing start，hovered a few feet above the ground，went sideways and backwards，and then settled back to earth．That vehicle Was a helicopter． Imaginations were fired．Men dreamed of commuting to work in their personnel helicopter．Every man would have one in his backyard．People anticipated that vertical flight transports would carry millions of passages as do the airlines of today．Such fantastic expectations were not fulfilled． The helicopter has now become an extremely versatile machine．It excels(出色完成)in military missions，carrying troops，guns and strategic instruments where other aircraft cannot go．Corporation use them as airborne(空中的)office，many metropolitan areas use them in police work，construction and logging companies employ them in various advantageous ways，engineers use them for site selections and surveys，and oil company use them as the best way to make offshore and remote work stations accessible to crew and supplies．Any urgent mission to a hard-to-get-to place is likely task for a helicopter．Among their other multitude(大批，众多)of uses，they deliver people across town，fly to and from airports，assist in rescue work，and aid in the search for missing or wanted persons． Under what conditions are helicopters found to be almost indispensable