单项选择题
A．the skill of the scientist B．the extent to which variables can be controlled C．the requirements of the observer D．the development of more accurate instruments
A．on aviation history B．about a man called Lindbergh and his plane C．about a famous flight across the Atlantic D．about how to go to Paris from New York by plane
A．the airfield was wet B．he took too much fuel with him C．a tractor almost blocked the runway D．it was too dark to see well