44.I’m afraid you must have met with many difficulties in th

答案应该是didn't you
一、对现在情况的推测,问句部分用主动词（do,be)一般现在时的适当形式.若是现在进行时,问句部分用现在进行时的适当形式表示.若是there be结构,问句用isn't/aren't there.如：
1)He must be there,isn't he?
2)He must have a big family,doesn't he?
3)He must be waiting outside,isn't he?
4)There must be some students in the room,aren't there?
二、对已发生的过去情况的推测,若陈述句谓语部分有“must have done”,而且有表示过去的时间状语,问句部分用didn't；若没有表示过去的时间状语,问句部分用haven't或hasn't.如：
1)They must have gone there last night,didn't they?
2)They must have arrived by now,haven't they?（根据by now来判断）
3)They must have been to the Great Wall,haven't they?
三、若是被动,应按被动结构来处理.如：
1)The room must have been cleaned yesterday,wasn't it?
2)The room must have been cleaned,hasn't it?
四、若句中有表示过去完成时的时间状语,问句部分应用hadn't.如:
They must have learnt 5000 English words by the end of last term,hadn't they?
(本题中must表推测,如果将它去掉,还原为真实句就是They had learnt 5000 English words by the end of last term.因此,反意问句是hadn't）

这是固定的句式好象没有什么原因的，mustn't只是几种特定的情况才选

mustn’t 是禁止的意思

当must用来表示对过去的情况进行“推测”（must + have done）时，如强调对过去情况的推测（一般句中有过去的时间状语），反意疑问句部分要用“didn’t + 主语”；如果强调动作的完成（一般没有过去时间状语），反意疑问句部分要用“haven’t / hasn’t + 主语”。
例 She must have read the novel last week, didn’t ...