用所给词的适当形式填空。

1. When is ______ (sally) birthday party?

2. December is the ______ (twelve) month of a year.

3. Mary's birthday is August _______ (eight).

4. Look at the photos of these famous _______(people).

5. There are twenty _______ (class) in our school.

一瓶固体经化验只含有一种元素，则该固体是（　　）

A．一种单质

B．单质与化合物的混合物

C．一种化合物

D．既可能是一种单质，也可能是混合物