20. If you are weak _____ a subject, ______ and asking a teacher for advice can help.
   A. at; working hard               B. in; work hard 
C. at; work hard                  D. in; working hard
21. While flying, a bat makes a sound which hits things and comes back to its ears. In this way it’s able to know ______ the things are and avoid bumping into them.
A. how    B. where        C. what         D. which
22. If we continue to be ______ the problem, things are sure to go from bad to worse.
A. blind to     B. interested in    C. honest about  D. satisfied with
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