The number of primary schools in the rural areas _____ doubling in the past 50 years.
A. are
B. is
C. have been
D. has been
A. are
B. is
C. have been
D. has been
【答案】
A. are
B. is
C. have been
D. has been
A. are
B. is
C. have been
D. has been
【答案】
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