题干
________ London many times, but he knows little about the city.
A：Having been to
B：Though he has been to
C：He has been to
D：He has gone to
答案(点此获取答案解析）
C
A：Having been to
B：Though he has been to
C：He has been to
D：He has gone to
,
,
是单位向量，且
,那么
的最大值为