--Who helped you clean the classroom yesterday? . I cleaned it all by myself. A. Somebo

问题补充：

--Who helped you clean the classroom yesterday?

-- . I cleaned it all by myself.

A. Somebody B. Nobody C. Everybody

答案：

解析：四个选项分别表示一些人、没有人和每个人，根据下文前面应为否定含义，故选B，没有人。我自己做的。