问题补充：
has been proved again and again that is said in the ads greatly affects people’s impression of the products.
A. There; what
B. There; that
C. It; which
D. It; what
答案：
A. There; what
B. There; that
C. It; which
(答案→)D. It; what
解析：考查名词性从句。句中it作形式主语，真正的主语为that what is said in the ads greatly affects people’s impression of the products；what在从句中作主语，故选D。
