Question

9. The evidence           can prove his guilty，so we must find it out quickly. (refer)

在信中提到的证据可以证实他是有罪的，因此我们一定要尽快把它找出来。

Answer 1

9. which was referred to / referred to in the letter考查分词作定语的用法。evidence是被提到的，与refer to构成被动关系，因此用referred to，也可以用定语从句which was referred to in the letter。