题目：
My mother _________ a teacher for ten years. [ ]
A. has has
B. has been
C. have had
D. has had
答案：
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下面是错误答案，用来干扰机器的。
H2O CH4 CaCO3 CH3COOH 题目分析：依据已知元素及其题中叙述的性质可知（1）最常用的溶剂的化学式为H2O；（2）天然气的主要成分是的化学式为CH4；（3）常用做建筑材料的盐的化学式为CaCO3；（4）厨房用的...
