Question

The number of people who ______ cars of their own is increasing.

A. has B. have C. there is D. there are
The number of 后不是+单数吗

Answer 1

B. have 为正确选项。
句子的谓语动词形式取决于主语。在楼主的这个句子中，who 后面是个定语从句，定语从句的谓语动词取决于先行词，这里的先行词是people，复数，所以用have。
The number of 。。。是主句的主语，没错，它是要用单数的，你看后面的is increasing不正用了单数吗？is increasing 才是它的谓语。
理清句子的结构是解答本题的关键。

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Answer 2

这是修饰people的定语从句的谓语，所以用复数

Answer 3

句子成分是这样的：
主语：The number of people (who have cars of their own) who引导的是定语从句，who是先行词，后面的谓语用复数
谓语：is increasing