As a war reporter, Jenny went through a lot of adventurous __________, which contributed to her rich _______ in her job.

A．experience; experience

B．experiences; experience

C．experience; experiences

D．experiences; experiences

在匹配病例对照研究时，为了节约样本，提高效率，常用1∶A4配比，但A4的取值一般（）。

A.<2

B.<3

C.<4

D.<5

E.<6