My aunt is busy,she has to look after her baby.It's only three months old.为什么用“it”?

My aunt is busy,she has to look after her baby.It's only three months old.为什么用“it”?
My aunt is busy,she has to look after her baby.It's only three months old.为什么用“it”?

My aunt is busy,she has to look after her baby.It's only three months old.为什么用“it”?
it是指代 前面的her baby.因为小孩较小可不太区分性别所以有此用法.
注意第5点
简简单单一个代词IT真是神通广大,用途多多,其功力比其他任何代词都庞大.
从中文立场看,好些无主语的句子,在译成英文时,都非请IT协助不可.例如：
①是这样!（That's it!）
②没办法.（It can't be helped.）
③轮到我了.（It is my turn.）
④事实上,我们可以打败他们.（As it is,we can beat them.）
⑤离开这里不远.（It is not far from here.）
⑥只要一块钱.（It is only one dollar.）
⑦这儿真热.（It is so hot here!）
此外,“it”还有下列五种用法：
⒈代替上文提到的事物或情况,如：
Look up the word in the dictionary if it is difficult to you.
I would appreciate it if you could do me a favour.
⒉代替没生命的东西,如：
Here is a plastic box.It is useful.
Don't throw the old newspaper away; you may need it.
⒊代替普通动物,如：
The dog outside belongs to the owner of the house.It is very fierce.
Look at the cat!Isn't it furry?
指大动物,如马、大象等或指人们心爱的宠物,如猫、狗等时,可用“he”或“she”,而不用“it”；故事中的动物便是个例子,如：
When the children saw a horse coming,they stopped and looked at him.
A cow is grazing grass in the field.She looks very strong.
⒋用来指天气、日期、季节、时刻、距离等,如：
It is raining outside.Don't go out!
It is the 20th of December.
Time flies!It is vacation again.
It was half past five when Jason got up.
It is very far from the school to the market.
It is only a short distance to the library.
⒌可以指性别不详的婴儿,及已知的事实,如：
The baby next door kept me awake.It cried loudly.
Yes,I was at home last night.What about it?
When the company closes down,it (i.e.this event) will affect some staff's future.
⒍可以用来强调句子的某部分,如：
I respect David.It is he that has made the department what it is now.（强调主语）
It is Mandarin that Janet has failed to pick up.(强调宾语)
It is only recently that she has embarked on the learning of Mandarin.（强调副词“recently”）
从上述例子看,代词IT的功用,不仅在代替单数的事物和动物等,还可以协助强调句子成分以及在中英翻译中协助中文无主句的英译.

因为前一句有出现her baby了,再在下一句出现就显得烦琐,所以就用it代了

这里的it 是做代词用的！因为前面已经提到了这件事！所以后面没有必要再重复一下！因此用it代替前面说过的事情！

it是主语

根据it前面的"three years old"可以知道绝对不是说前面所说的事情，应该是说her baby吧，英语中偶尔也会出现用it 代指人的情况，会出现在口语中

it 就是"它"啊,不能没有主语吧