单选题 Stock farmers must be ______ to the possible reoccurrence of foot-and-mouth disease that has already killed millions of pigs and cows in East Asia.
【正确答案】 C
【答案解析】[解析] 词汇：形容词
1．这是一道词汇题，选项均为形容词。
2．空格后提到口蹄疫复发，符合这一语境的为alert，意为“警戒”。