题目

So far they __________ to speak four languages. Sometimes they _________ them. A. have learned; have mixed                           B. have learned; mix C. learned; mix                                          D. learned; mixed 答案：B2．短周期主族元素X、Y、Z、W原子序数依次增大，X的质子数与电子层数相同，Y、Z同周期且相邻，Z的最外层电子数是其内层电子数的3倍，元素W在地壳中的含量仅次于氧．下列说法正确的是（　　）A．原子最外层电子数由多到少的顺序：Z＞Y＞W＞XB．X、Y、Z三种元素形成的化合物中只有共价键C．简单气态氢化物的热稳定性：Y＞ZD．最高价氧化物对应水化物的酸性：W＞Y