没有正确选项，C是打错吧，应该是much more diffictult 或 more difficult ,要不就是原句多了than
即：
this math problem is （C . much more difficult / more difficult ) than that one
或
this math problem is ( B as diffictult as ) that one.
加油！不明白再问！如果帮到你，请采纳，谢谢！
D，这道数学题没有那道简单
so+形容词原级+as，一般用于否定疑问句
as+形容词原级+as
much+形容词比较级+than
less+形容词比较级+than
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