To their surprise, they found the lecture given by the famous professor easy .
A. to be understood
B. to understand
C. understanding
D. understood
网友回答
A. to be understood
(答案→)B. to understand
C. understanding
D. understood
解析：句意：使他们吃惊的是，他们发现这个著名的教授做的讲座很容易明白。此题考查不定式修饰表语的形容词作状语，easy后跟不定式，用主动表被动，所以选B。