A、b/t≤13 <img src='https://tiw.cn/files/2015-10/liuziyi/2015102810372055880.jpg'>
B、b/t≤18 <img src='https://tiw.cn/files/2015-10/liuziyi/2015102810372055880.jpg'>
C、b/t≤40 <img src='https://tiw.cn/files/2015-10/liuziyi/2015102810372055880.jpg'>
D、b/t≤15 <img src='https://tiw.cn/files/2015-10/liuziyi/2015102810372055880.jpg'>
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