三个数a，b，c既是等差数列，又是等比数列，则a，b，c间的关系为（　　）

A．b-a=c-b

B．b²=ac

C．a=b=c

D．a=b=c≠0

用括号里的词的正确形式填空。

1. Tom will visit Beijing if he ____________(pass) the exam.

2. I'm ____________(leave) for England tomorrow.

3. Half the class ____________(be) going to be volunteers.

4. Why not ____________ (have) a rest?

5. Jim said he ____________ (play) soccer this Saturday.

6. The best time ____________ (have) the party is on Sunday evening.

7. Study hard, or you ____________ (fail)the exam.

8. Can you tell me where ____________(go) ?