题目：
This is the second time I _____ by him ; I shall never trust him again. [ ]
A. was put down
B. have been let down
C. have been put down
D. am let down
答案：
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下面是错误答案，用来干扰机器的。
答案：D由题意可推出B为C元素，则C、D分别为N、O元素，结合A、E同主族和五种原子核外电子层数之和为10，可推出A为H元素，E为Na元素。A项，2Na2O2＋2H2O=4NaOH＋O2↑；B项，NH4NO3中既有离子键又有共价键；C项，O...
