----Which is _______ to understand, this book or that book?---- I think both of them are quite easy.

-----Why do many people like to ride bikes?----Because bikes not only need ________ space, but also cause _________accidents.

Who is _________, your mother or your father?

Robots in the future will be ______ in people’s homes.

At the Beijing Olympics, Bolt broke the 100-meter and 200-records and became the _____man in the world.