Friendship is like money, easier made than ________[ ]

So difficult _____ it to live in an English-speaking country that I determined to learn English well.

_____ any other species, human populations are shaped by the usual forces of natural selection, like famine, disease or climate[ ]

As one friend of his, you should give him a hand when he is in trouble.

We can hardly believe that an intelligent person like Daniel _____ be asking such a silly question. [ ]