Which single factor is tended to have been considered by the author as the most important in the development of man

A．They don’t enjoy swimming.
B．They don’t know how to swim.
C．They’ll swim in. the lake tomorrow.
D．They won’t go swimming in the lake today.

A．He was annoyed.
B．He was pleased.
C．He felt it could be replaced.
D．He did not care.