21. (2018宜宾)It's getting dark. You'd better________ out by yourself. It's dangerous.
A. not go  B. not to go  C. to go
21. A【解析】考查省略to的动词不定式。句意：天黑了，你最好不要独自出门了，那太危险了。had better not do sth.最好不做某事。故选A。