John has put forward _______ most challenging question for us to answer.-Yes, it really is. I have never heard _______ harder one.

On his way home yesterday , Tom was hit onthe head_____he had time to realize what was happening .

Space exploration began in the 1960s, and since then about five hundred people _______ into space. [ ]

________it was a waste of time and energy，but she didn’t accept my advice.

As someone ______ the curtain, the famous singer came out. The fans ______ and screamed with excitement.