It is believed that a new kind of drug ______ by the scientists and they are hopeful that they will succeed in a few months. 

Mary said they _____________________with each other for six years before they got married.

Recently quite a lot of experts have suggested that another law on wildlife protection      as soon as possible．

When the old man ______ to walk back to his house, the sun ______ itself behind the mountain. 

 Ladies and gentlemen, please fasten your seat belts. The plane ______..