21. A study ________ in the U.S.A. in 1977 showed that the chemical ASA in aspirin could prevent a stroke.

   A. carrying out           B. carried out      C. working out   D. worked out

22Penicillin was then produced ________ in order to meet the demands of the armies.

     A. in large amounts                                  B.    in large plenty

     C. in large quantities                               D.    in large number

23.The little girl ________ a cry at the sight of a big snake.

     A. let out      B. gave out           C. made out           D. shouted out