_____is cool today

选B
IT 的用法
1.做代词,代替刚提到的过的一件事情.
a.可以指一个具体的东西.b.可以指前面所谈的事情或情况.
eg.a.It’s a nice room.
b.You promised to write the article,and you must do it.
2.做代词代替指示代词 this,that
eg.---What’s this?------ It’s a flag.
3.起指示代词的作用,指一个人或事物,it 所指的东西不很具体.
a.有时指某个动作的人.b .有时指引起某种情况的事物.
eg.a..----Who is knocking at the door?---- It’s me.
b.It’s the wind shaking the window.
4.指环境,情况等.
eg.It’s very quiet at the moment.
5.指自然现象（天气,气候,明暗等）
eg.I’s getting cold (dark,late,etc.).
6.指季节,时间
eg.It was late autume (early spring,mid summer,etc).
7.指距离
eg.It’s only five miles (half an hour’s walk).
8.用于强调结构,在这里it 可以说没有意思.它只帮助改变一个句子的结构,使某一成分受到强调,改变结构的办法是：
IT + be + 要强调的部分+ that(who,whom) + 句子其他部分
强调的部分是人用who(m),其他情况多用that
eg.It was Mary who (that) met your sister in the zoo yesterday.
It was your sister that (whom) Mary met in the zoo yesterday.
It was in the zoo that (where) Mary met your sister yesterday.
9.做形式主语,代替一个由不定式,动名词短语或是从句表示的主语,使原来的这些主语可以放在句子后部,避免头重脚轻.
a.真正的主语是不定式.
Eg.It’s our duty to attend to this letter.
b.真正的主语是动名词.
Eg.It’s no use talking to him about it.
c.真正的主语是从句,这个从句可以用that 引起,也可以用一个连接代词或连接副词引起.
Eg.It happened that I wasn’t there that day.
It’s doubtful whether she will be able to come..
10.做形式宾语,代替一个由不定式,动名词,或是宾语从句,往往把宾语放在它的补足语后面.而用it做形式宾语,放在宾语补足语之前.
Eg.I think it no use arguing with him.