21．________swallow（燕子）doesn’t make ________ summer.
A. One; a B. One ; ／ C. A; the D. A; ／
22．Use your head. nothing taught by others can have the same effect on you as ________learned by yourself．
A. those B. what C. that D. the one
23．— You ought to have come to see Alice yesterday.
— Yes, I know I________.
A. must have B. could have C. ought to D. should have
