A.cold viruses
B.scientists
C.people
D.common colds

A.often caught colds
B.never caught colds
C.suffered a lot
D.became very strong

A.I didn't visit Canada two years ago.
B.I have paid a visit to Canada since last year.
C.I have been in Canada for two years.
D.I haven't come back to Canada for two years.

A.I didn't visit Canada two years ago.
B.I have paid a visit to Canada since last year.
C.I have been in Canada for two years.
D.I haven't come back to Canada for two years.

A.The father asked Tom to play with the knife.
B.The father warned Tom not to play with the knife.
C.The father accused Tom of getting a cut.
D.The father ordered Tom to get a cut.

A.The athletes might have practiced hard for the games.
B.The athletes must have practiced hard for the games.
C.The athletes should have practiced hard for the games.
D.The athletes couldn't have practiced hard for the games.

A.In
B.polite
C.had stood up
D.entered

A.In
B.polite
C.had stood up
D.entered

A.enjoy
B.coaster ride
C.the most terrifying
D.of their

A.were working
B.the teacher
C.around
D.peering