It is reported that no problems have been discovered in milk products such as Yili Sanlu

问题补充：

It is reported that no problems have been discovered in milk products such as Yili, Sanlu and Mengniu, proves newly-produced liquid milk safe.

A. that

B. what

C. which

D. where

答案：

A. that

B. what

(答案→)C. which

D. where

解析：考查定语从句，which引导定语从句指代前面的句子，并且在定语从句中做主语。