下列四个命题：

①f（a）f（b）＜0为函数f（x）在区间 （a，b）内存在零点的充分条件；

②命题“若x²＜1，则-1＜x＜1”的否命题是“若x＞1或x＜-1，则 x²＞1”；

③正弦函数关于X轴对称．

④正切函数在定义域是单调的．

其中真命题的个数为（　　）

A．1个

B．2个

C．3个

D．4个

词汇运用。

1．Tom lives on the _______ (九) floor while I live on the top floor.

2．He held a party without _______ (邀请) us.

3．These stickers are expensive. I need a much _______ (便宜的) one.

4．The air _______ (污染) is very serious in that city.

5．— Are these dogs on the road those _______ (邻居)? — No, they are ours.

6．There are some children _______ (ride) horses.

7．Tom, _______ ( not play) on the road! It's dangerous.

8．He spent less time ________ (design) the project than I.

9．Would you like _______ (choose) the nicest picture for your teacher?

10．Please don't make the boy _______ (cross) the street by himself.