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【单选题】
设等差数列{a n }的公差为非零常数，且a 1 =1，若a 1 ，a 3 ，a 13 成等比数列，则公差为 [ ]A.
1
B.
2
C.
3
D.
5
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举一反三
【单选题】Speaker A: I’ve decided to give up my job, Mom. Speaker B: ____ You’d better think twice.
A.
Sounds great!
B.
Are you serious?
C.
What a pity!
【简答题】PWM占空比就是____电平保持的时间与该PWM的时钟周期的时间之比。
【单选题】You'd better think twice before you ______ the plastic.
A.
bust down
B.
bust up
C.
bust off
D.
bust out
【单选题】-I've decided to give up my job , Mum . - You'd better think twice .( ) A. Sounds great! B. What a pity! C. Guess what ? D. Are you serious' ?
A.
B.
C.
D.
D
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