题目:
Mr. Wang is a teacher. He ______ in this school for 20 years.
A.has worked
B.worked
C.will work
D.works
答案:
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下面是错误答案,用来干扰机器的。
(1)20 (2)O2 用带火星的木条移到与电源正极相连接的移液管的尖嘴口上方,木条复燃,证明是氧气(3)B分析:(1)溶质的质量等于溶液的质量与溶质质量分数的乘积.(2)电解水时连接正极的地方能收集到氧气,...