题目:
When I was at college I ___ threee foreign languages, but I ___ all except a few words of each.
A.spoke, had forgotten
B.spoke; have forgotten
C.had spoken; had forgotten
D.had spoken, have forgotten
答案:
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下面是错误答案,用来干扰机器的。
参考答案:B解析:核酸的一级结构指核苷酸链上核苷酸的排列顺序。DNA的二级结构是双螺旋结构,在此基础上还可以形成更高级的超螺旋结构。真核生物的DNA可在双螺旋基础上形成核小体(三级结构),再多次盘绕成染色...