The population of Africa is so rapidly as to cause concern of the whole world.
A. expanding
B. circulating
C. exporting
D. spreading
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(答案→)A. expanding
B. circulating
C. exporting
D. spreading
解析：动词辨析。句意：非洲人口在快速增长，引起了世界的关注。expand：扩张、发展；circulate：循环；export：出口；spread：扩散。