We must be ______ our guard against this kind of disease.

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问题 We must be ______ our guard against this kind of disease.

选项 A、on
B、in
C、with
D、for

答案A

解析 be on one’s guard against是固定词组,意思是提高警惕。注意:be on guard,be on alert都指警惕,提防。
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