焊接设备的电焊把线应采用绝缘良好的橡皮软导线，其长度不应超过（）m。
A.20
B.30
C.40
D.50
参考答案：D
氢原子能级的示意图如图所示，大量氢原子从n＝4的能级向n＝2的能级跃迁时辐射出可见光a，从n＝3的能级向n＝2的能级跃迁时辐射出可见光b，则
A.氢原子从高能级向低能级跃迁时可能会辐射出γ射线
B.氢原子从n＝4的能级向n＝3的能级跃迁时会辐射出紫外线
C.在水中传播时，a光较b光的速度小
D.氢原子在n＝2的能级时可吸收任意频率的光而发生电离
It is said that a grave of a middle aged man ______ about 2,300 BC was found by some English archaeologists in the year of 2002.
A．dating back B．dated back toC．dating back to D．dated from
