It has been announced that all the students ____ put on masks before going to school in case they are infected with SARS virus.

You promised your friend a letter; you ought to ______ days ago.

The 2010 World Expo _____ in Shanghai isexpected to attract more than 70 million visitors from home and abroad

The police are to ______ the traffic accident that we don’t know _____.

Though I drank too much,I drove back home .