My tape-recorder is ________.

as good as 是“和……一样好”的意思,句中as good as后面没有加比较的对象,而是加了逗号跟了一个条件状语if not better than,之后的Anne's本应是接在as good as 后面的,所以加逗号隔开区分.更容易理解的表达是：If my tape-recorder is not better than Anne's,it( my tape-recorder) is as good as Anne's.
再说B选项,如果没有后面的逗号的话,表达是不完整的：缺少as good as 的比较对象~

if not better than是一个插入，也是个省略句，外国人喜欢省略所以有些没那么规范和严整。但as good as后面就得接个明确对象了，Anne's 其实是接是接前面as good as后面的，这个句子真正的顺序是My tape-recorder is as good as Anne's,if (it's) not better than (hers）

你是不是打错了，A、B 一样啊？