Which sentence from the passage supports the idea that the "world is chiefly indebted" to James Watt and George Stephenson for developing the steam engine?
A.
The first train consisted of thirty-four cars weighing, all told, ninety tons.
B.
Watt made it a quick, powerful, and efficient engine, requiring only a fourth as much fuel as before.
C.
Before the time of Watt, the steam engine was exclusively a steam pump—slow and wasteful of fuel.
D.
The idea of using steam for driving stationary machinery originated in the early centuries.