These are questions ______ by history.

For many years, people________ electric cars. However, making them has been more difficult than predicted.

The expert warns the Internet users not to ______ their personal information to strangers.

We students are supposed to know how we should ourselves to surewe are in the proper mental state.

--- Believe it or not, I ______ an old friend of mine while abroad.--- Oh, it’s a small world.