Because Robert was seen the room everyone thought he was the thief.A. to enter B. enter

问题补充：

Because Robert was seen the room, everyone thought he was the thief.

A. to enter B. enter C. Entered

答案：

解析：句意为：罗伯特被看见进入了房间，大家都认为他是小偷。这是一个由含有宾语+不带to的动词不定式作宾语补足语的句子变成的被动句。原句是Everyone saw Robert enter the room.

变成被动句时宾语补足语的动词不定式要加上to。故应选A。