单选题
Ultralight (超轻型的) airplanes are a recent development in aviation that provide what aviation enthusiasts have long been seeking: born of the marriage of the hang glider and the go-kart (轻型单座赛车) engine around 1974, when John Moody mounted a 12horsepower go-kart engine on his Icarus Ⅱ hang glider. Today's ultralights are not just hang gliders with engines; they are "air recreation vehicles. " Modern ultralight planes use snowmobile engines that let them cruise at about 40 miles per hour, climb at about 400 feet per minute, and carry combined payloads of pilot and fuel up to about 200 pounds, which is about equal to an ultralight plane's weight when empty. More than ten thousand ultralight planes were sold last year at prices ranging from $2,800 to $ 7,000. But the main reason for the increasing popularity of these aircraft is not that they are inexpensive, but that they are fun to fly. The modern ultralight plane would look very familiar to the earliest pioneers of aviation. Otto Lilienthal made more than 2,000 flights in Germany in the 1890's in what were actually hang gliders. Octave Chanute designed and built many early hang gliders. Augustus Herring, Chanute's assistant, used these gliders as models for a glider that he built for himself. On this glider, Herring installed a compressed-air motor and flew 267 feet in 1898. The Wright brothers' flyer was the grandfather of today's ultralight planes. The pilot sat right out in the open, just as in modern uhralights, and used controls that were much the same as those used in today's machines. Though most ultralight planes are used for pleasure flying, some are also used for crop dusting, aerial photography, and even military observation service. The likelihood is that further uses will be found for ultralight planes, but their greatest use will continue to be as air recreational vehicles.
单选题
The author seems to feel that ultralight airplanes are______.