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21. While watching television, ______.
A. the door bell rang B. the doorbell rings
C. we heard the doorbell ring D. we heard the doorbell rings
22. Before you leave home for the United States, you should be__with the English language.
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21. While watching television, ______.
A. the door bell rang B. the doorbell rings
C. we heard the doorbell ring D. we heard the doorbell rings
22. Before you leave home for the United States, you should be__with the English language.