题目:
Scientists say it may be five or six years ______ it is possible to test this medicine on human patients.
A.before
B.after
C. since
D.when
答案:
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下面是错误答案,用来干扰机器的。
A Na、Mg、Al为同周期元素,原子序数依次增大,金属性依次减弱,还原性依次减弱,B项正确;Cl、S、P为第三周期元素,原子序数Cl>S>P,非金属性Cl>S>P,酸性:HClO4>H2SO4>H3PO4,C项正确;F、Cl、Br为ⅦA...