21. In 1948, three American scientists invented ____ transistor, which is ____ small electronic device to
control an electric current.
A. the; a B. a; a C. the; the D. a; the
22. — Has Yang Liping returned from Yunnan yet? I’d like to meet her.
— Sorry. She ____ back here only for last weekend.
A. has been B. would be C. had been D. was
