求解答Prep2012的语法题

elginlv

Prep2012-Pack1-SC-052 VSC002367 Medium

The industrial pollutants known as were first manufactured in 1929 and were used as coolants for electrical equipment in Europe and North America the 19705, when studies showed that the compounds had entered the food chain and were harmful to some animals. A

A. that the compounds had entered the food chain and were harmful to some animals

B. the compounds to have entered the food chain and be harmful to some animals   

C. the entry of the compounds into the food chain as harmful to some animals   

D. the entry of the compounds into the food chain and its harmfulness to animals   

E. the compounds entering into the food chain and harming some animals   

我原本选的是B。我明白为何B是错误的，但我还是不明白为何A是正确选项。

我什么要用past perfect （had entered）呢？

elginlv

自己顶一个！！！！！！！！！！！！！！！！！！

xiqiong

时态不做主要的考察。一般遵循原句。这里必须要有that，要不会有歧异。