The number of primary schools in the rural areas__________ doubling in the past 50 years.<br />A.are<br>B.is<br>C.have been<br>D.has been

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

出自：国家开放大学英语语法

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