碱基替换是基因突变的重要分子基础之一。碱基颠换指的是()
A.嘌呤替换嘌呤
B.嘧啶替换嘌呤或嘌呤替换嘧啶
C.嘧啶替换嘧啶
D.碱基类似物替换正常碱基
参考答案:B
24.—I just heard you had an adventure in the desert.
—We lost our way and ______ made matters worse was that our jeep broke down.
A.how
B.that
C.it
D.What