出口食品生产企业“卫生注册证书”的有效期是（ ）年，“卫生登记证书”的有效期是（ ）年。

A．3；5

B．3；3

C．5；3

D．5；5

We are taught that a business letter should be written in a formal style （） in a personal one.

A. rather than

B. other than

C. better than

D. less than