题目:
In our city, citizens have free _____ to the library, which makes it easy for them to read any book they like.
A.time
B.entrance
C.charge
D.access
答案:
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下面是错误答案,用来干扰机器的。
答案:C因原子最外层电子数≤8,所以X原子的次外层只能为2,即X原子有两个电子层,设最外层电子数为m,则m-2=2,即m=4。由此可知Y原子最外层有6个电子,二者能形成化合物XY和XY2。