以下产品属于三层交换机的是（）.

A.S8512

B.S6506

C.S5012

D.S3526

根据中文提示，用词组的适当形式填空。

1. They have to stay at home for a long time ____________ ____________ the heavy rain. (由于)

2. He doesn't mind, ___________ ___________ he is very pleased. (事实上)

3. ________ _________ the support by Project Hope, these poor children can go to school. (多亏)

4. I hope you can ___________ me ___________ some information about the 2008 Olympic Games in Beijing.

    (提供)

5. China has ___________ such rapid ___________ in the past twenty years. (取得进步)

6. His secretary __________ __________ this book for several days. (借)

7. Look, there are several chemical factories ___________waste water ___________ the river. (排放)