本题题意为 为了
保护动物，我们必须保护他们生存的地方
这里用 to do 作主语 表目的
to do和 doing 作主语的区别
1）一般来说，表示比较抽象的行为动作时多用
动名词doing；表示比较具体的某次行为动作时多用不定式to do。如：
No smoking is permitted here.
It is not very good for you to smoke so much.
Reading more is necessary.
It is quite necessary to read it again.
2）在许多情况下，两者可以互换，意义上没有太大的差别。如：
Seeing him happy made me happy too.
It made me happy to see him happy.
It is useless learning something without practising it.
It is useless to learn something without practising it.
3）不定式to do作主语时，常使用
形式主语结构；而动名词doing作主语时，较少用形式主语结构，只有在 It is no good, It is no use, It is useless, It is worthwhile 等句型中出现 it 作形式主语现象。如：
It is hard to make him change him mind.
It is not an easy thing to master a language.
How would it be to start tomorrow?
It took all his strength to move out of the crowds.
It seemed a pity to give up the chance.
Business is business and it will not do to be so careless.
It is no good pretending to know what you don't know.
It is no use regretting the past.
It is worthwhile learning another language.
4）在 there is no ... 句型中的主语只能用动名词doing，并且不能带逻辑主语。如：
There is no denying the fact.
Well, you have said it and there is no taking it back.
上述句子的意思如用不定式to do 则分别为：
It is impossible to deny the fact.
Well, you have said it and it is impossible for you to take it back.
5）doing 作主语时强调的是做那件事的状态，而to do 作主语时主要表示
一般现在时或者是将来时的事情
它们的具体区别 语法书上都有的 希望我的回答可以帮到你