一道语法题目

South Korea has witnessed the world's most dramatic growth of Christian congregations:church membership is expanding by 6.6 percent a year,fully two-
thirds of the growth coming from conversions rather than the population
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increasing.
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a.coming from conversions rather than the population increasing

b.coming from conversions rather than increases in the population

c.coming from conversions instead of the population's increasing

d.is from conversions instead of the population's  increases

e.is from conversions rather than increasing the population

the answer is b,i wander why not choose d.

因为未划线部分有church membership is...，这样造成两个独立的句子之间没有连词。