【单选题】We must be ______ our guard against this kind of disease.
[A] on
[B] in
[C] with
[D] for
网考网参考答案：A 网考网解析： [名师精解] be on one's guard against是固定词组，意思是提高警惕。 [解题秘籍] be on guard，be on alert都指警惕，提防。
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