I had to buy        of these books because I didn’t know which one was the best.

All 　　is needed is a supply of oil.

Everyone knows that _______ is dangerous to play with fire, but _______ is difficult is to prevent children from playing with fire.

I’m moving to the countryside because the air there is much fresher than ________ in the city.

The film suddenly jumped from the events of 1920 to ____ of 1930.