What were you doing when she dropped in—I for a while and_________ some reading.

It’s a duty for friends to ________ each other when they are in trouble.

the first time you _______ Beijing—-No. But the first time I ______ here, the citywasn’t so beautiful.

As we all know, great changes _______ in China in the past thirty years.

_____ important for us _____wild animals and birds.