题干
―We didn’t see Robert at the opening ceremony yesterday.
―No, he _______ it.
―No, he _______ it.
A：mustn’t have attended
B：needn’t have attended
C：needn’t attend
D：didn’t need to attend
答案(点此获取答案解析）
D
,向量
,则
的最大值为（ ）．