受精卵分裂发育成胚胎，胚胎发育的主要场所是   [ ]

A．子宫

B．卵巢

C．输卵管

D． * *

—Hi, Andrew. How can you improve your English so much?

—Oh, nothing difficult. The ____ you work at it, the ____ progress you will make.[ ]

A. harder; more                    

B. more hardly; more

C. hardlier; greater                  

D. harder; great