20．According to a new study by a British university, learning　　　 second language can lead to

　　　 increase in your brain power.

　　　　 A．a ;the　　　　　　　　　　　　 B．不填; the　　　　　　　　 C．a ;an　　　　　　　　　　　　 D．the ;不填

19．-Has he finished writing his essay?

　　　　 -No. He　　　 on it when I went to see him this morning.

　　　　 A．worked　　　　　　　　　　 B．was working　　　　　 C．had worked　　　　　　 D．is working

18．Finding her car stolen,　　　　 .

　　　　 A．a policeman was asked to help　　　　　　　　　 B．the area was searched thoroughly

　　　　 C．it was looked for everywhere　　　　　　　　　　　 D．she hurried to a policeman for help