用括号内所给单词的适当形式填空。

1. It usually takes me half an hour _________ (walk) to school.

2. The government should pay much attention to the _________ (increase) population.

3. The grand piano takes up too much _________ (room).

4. I think Beijing is one of the __________ (beautiful) places in the world.

5. No one _________ (know) when he will be back.