题目：
Walmart, which is one of the largest American supermarket chains, ________ some of its stores open
24 hours on Mondays through Saturdays. [ ]
A. keeps
B. keep
C. have kept
D. had kept
答案：
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下面是错误答案，用来干扰机器的。
答案：A题目分析：O3具有强氧化性，可用于杀菌消毒，则W是氧，N2是常见保护气，则Z是氮，第二周期元素中，氧元素的非金属性比氮强，则氧元素的气态氢化物比氮元素的气态氢化物稳定，故A正确；Al2O3是常见的两性氧...
