有一项关于吸烟与肺癌的关系的研究，研究者发现肺癌的吸烟比例为80%，非肺癌组的吸烟比例为50%，这是一项（）。

A.横断面研究

B.病例-对照研究

C.前瞻性队列研究

D.实验性研究

E.历史性队列研究

句型转换。

1. She can speak Chinese. (改为一般疑问句)

    __________ Chinese?

2. She is eating an ice cream. (改为否定句)

    She __________ an ice cream.

3. She has supper at home. (改为一般疑问句)

    __________ supper at home?

4. Tom is writing to his friends.  (对画线部分提问)

    Who __________?

5. Please send me a postcard. (改为同义句)

    Please send a postcard __________.