From William H. Pickering's quotation, we can conclude he thought the use of aero planes was ______.

A．Doubtful.
B．Scornful.
C．Critical.
D．Suspicious.

A．the hope that men could fly long distances through the air would remain a dream
B．it is clear that building a machine by which men shall fly long distances is impossible
C．all physical facts have demonstrated that men can not fly long distances through the air
D．men have not yet learned to produce a flying machine with known substances and known forms of machinery and force