参与Ⅰ型超敏反应的抗体主要是(①);参与Ⅱ型超敏反应的抗体是(②);在减敏治疗中,诱导机体产生的封闭抗体是(③)。
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At that time we thought the story_____ not be true.
A. could
B. must
C. can
You___ the work better in the future.
A. could did
B. can does
C. could do
The room is dirty. It ____ cleaned.
A. can‘t be
B. couldn’t be
C. mustn't be
Mary ____ the exam if she studied harder.
A. can have passed
B. could have passed
C. couldn't have passed