He visited three cities when he stayed in the U.S.，_____New York ,Washington and Houston.

He visited three cities when he stayed in the U.S.，_____New York ,Washington and Houston.

空格前面是逗号。
A 是列举
B 后面的简单句跟前面必须是并列或复合的关系才行，仅用逗号隔开是不可以的。是句号的话就Ok了 。
C 定语从句，which首字母不可大写，且定语从句的谓语动词的单复数取决于先行词（被修饰部分）。
D 正确，相当于 i.e. 或that is to say （即。。）

应该是,D吧。前问提到了三者是复数啊。

He visited three cities when he stayed in the U.S.，that is_____New York ,Washington and Houston.
他在美国逗留时参观了三个城市，这三个城市也就是纽约，华盛顿和休斯敦。
that is to say = that is, 是that is 的变体，
that is (to say):就是,即
(是解释前面的内容,在句中相当一个插入语,前面可以用逗号)

例如：
eg:
He is a local government administrator,that is (to say) a Civil
Servant.他是个地方行政官员,也就是说是文官.

like,such as 可以用来举例.
但我认为这句话并不是要举例,而是对前面的四种语言进行解释.

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