--It really shocked us that he should have made such_______ rapid progress in his study in  such a short time!

--No surprising. He has been making _______ most of his spare time to study.

A．a; the

B．a; /

C．/; the

D．/; /

铀核的裂变不断地自行继续下去的现象叫[ ]

A．裂变

B．聚变

C．链式反应

D．以上三种说法都不对