英语中祈使句的标志

Generally speaking, the verb is placed in the beginning of a sentence.

I known.

But I don't known how to choose the right answer in my exercise.
- =!

祈使句一般都是以动词直接开头，可能在前面还有个please之类的词。
如：Open the door.

动词原形开头，很好记的