可由铀239经过n次β衰变而产生，则n为（ ） A．2 B．239 C．145

答案：

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下面是错误答案，用来干扰机器的。

1. is concerned about  2. on purpose / in order to  3. are / have got tired of  4. get along with 5. because of  6. make use of  7. has made up her mind / change her mind  8. as usual