单选题 The advertisement for the security guard position says that applicants must be able to remain ______ for six hours straight.
A. alert B. observable
C. discovered D. obvious

【正确答案】 A
【答案解析】[解析] 选择恰当的词汇填入动词remain之后的补语的位置。remain后面主要接表示状态的形容词，作为招募警卫员的广告，填入空格为“警戒的”形容词最为恰当。