问题
选择题
He can swim very well, ______ he doesn’t want to join the club. [ ]
A. and
B. but
C. /
D. or
答案
答案：B
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He can swim very well, ______ he doesn’t want to join the club. [ ]
A. and
B. but
C. /
D. or
答案：B
则由关系R和S得到关系T的操作是