A number of visitors are enjoying paintings。
许多游客正在欣赏油画。
the number of 表示 “…… 的数量”，后面同样接可数名词复数，但是谓语动词要用单数
The number of visitors is eighty.
游客的数量是 80。
回到真题，in China 在中国，a number of people 许多人，who have their birthday 是定语从句修饰前面的 people，指过生日的人，cakes with candles 插着蜡烛的蛋糕，整句意思就是在中国，许多过生日的人会吃插着蜡烛的蛋糕。the number of candles 蜡烛的数量，the person's age 这个人的年龄，蜡烛的数量就是这个人的年龄。
解析
1. 第一空：a number of = 许多，中心词是 people（复数），谓语动词用复数 eat
定语从句 who have their birthday 修饰 people，不影响主句谓语。
2. 第二空：the number of = …… 的数量，中心词是 number（单数），谓语动词用单数 is
翻译
在中国，许多过生日的人会吃插着蜡烛的蛋糕。蜡烛的数量就是这个人的年龄。
专项练习题：a number of /the number of
一、单项选择（6 题）
1. ______ volunteers are planting trees. ______ the volunteers is 200.
A. A number of; The number of
B. The number of; A number of
C. A number of; A number of
2. A number of birds ______ singing in the tree.
A. is B. are C. was
3. The number of players in our team ______ 12.
A. are B. be C. is
4. ______ students in this classroom ______ from Brazil.
A. A number of; come
B. The number of; comes
C. A number of; comes
5. The number of new books ______ increasing. And a number of them ______ storybooks.
A. is; are
B. are; is
C. are; are
6. A number of ______ visited the museum last weekend.
A. tourist B. tourists C. a tourist
二、用 be 动词（is /are）填空（4 题）
1. A number of ducks ______ swimming on the lake.
2. The number of ducks ______ 35.
3. A number of messages ______ sent to me.
4. The number of messages ______ surprising.
三、句子改错（划出错误并改正）
1. The number of workers are 50.
2. A number of kid plays games after school.
