产生电子衍射的必要条件与充分条件是什么？
参考答案：
产生电子衍射的充分条件是Fhkl≠0，
产生电子衍射必要条件是满足或基本满足布拉格方程。
Mr Zhang _______ his cap and sat down as soon as he entered the room. [ ]
A. took off
B. took out
C. took away
D. took down
进行工频耐压试验，从（）因素考虑，选用移卷调压器比选用接触调压器好。
A.设备效率
B.设备功率
C.试验电流波形
D.试验电压波形
