单选题 We must be ______ our guard against this kind of disease.
  • [A] on
  • [B] in
  • [C] with
  • [D] for


【正确答案】 A
【答案解析】[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。