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问题 单项选择题

微分方程y"+2y'+y=0的通解为( )。

A．y=(C1+C2x)ex
B．y=(C1+C2x)e-x
C．y=(C1+C2)e-x
D．y=(C1+C2)ex

答案

参考答案：B

单项选择题

肺癌的临床表现为（）。

A.咳嗽，咳铁锈色痰

B.刺激性干咳，无痰

C.长期咳嗽，咳白色泡沫痰

D.咳嗽，咳血、咳大量脓痰

E.刺激性呛咳，痰中带血
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选择题

______ he is five years old, ______ he swims as well as his father. [ ]

A. Because, so

B. Because, /

C. Though, but

D. Though, /

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