It is exactly _____we behave _____has changed the world.  

It was at 8 o’clock ________ I went back home last night.

---Will it take me at least six hours to write this essay?---Yes, six hours ______ to write a good essay.

It was my sister        first had the idea to cycle along the river. 

It was not until dark _____ he found _____ he thought was the correct way to solve the problem.