The word" incessantly" ( Para 1 ) could best be replaced by ______.

A．The reason why the Wright brothers succeeded in manned flight.
B．The role the internal combustion engine played in the Wright brothers' experiments.
C．The Wright brothers' experience as pilots.
D．The importance of gliders to the development of airplane.

A．Because they allowed the Wright brothers to decrease the weight of their airplane.
B．Because they resulted in a three-axis control for their airplane.
C．Because they were important in the refinement of the wings for their airplane.
D．Because they used the data to improve the engine for their airplane.