用括号内所给单词的适当形式填空。
It usually takes me half an hour __________ (walk) to school.
The government should pay much attention to the __________ (increase) population.
The grand piano takes up too much __________ (room).。
I think Beijing is one of the ___________ (beautiful) places in the world.
No one __________ (know) when he will be back.