----Do you often use Baidu?          ----Yes, it’s one of ___________Internet search engines.

There is _____pollution and ____fishes in the river now than 20 years ago.

To make our city greener and cleaner, we need ______ trees.

Mum, this pair of shoes is too small. Would you buy me a ______ pair?

Of the two pens, the boy bought ______ expensive one at last.