句型转换，每空一词。

1. We went there in July. (对画线部分提问)

    ____ ____ you ____ there?

2. —____ is Hainan? (根据答语填词)

    —It is in the south of China.

3. It is a photo of China. (改为复数句子)

    ____ ____ ____ of China.

4. I met my teacher in the library yesterday. (改为一般疑问句)

    ____ ____ ____ ____ teacher in the library yesterday?

有三个关系R、S和T如下：

由关系R和S通过运算得到关系T，则所使用的运算为

A．笛卡尔积

B．交

C．并

D．自然连接