We must study English hard or we can't be in the library,(反义疑问句）

答案应该是can we?
如果单纯是出现了must,那么这个题目应该用needn't we?
详细用法参考下面
must 的三种反义疑问句
既然多数人感到麻烦我就来讲一讲must反义疑问句的用法.
你记住,must反义疑问句就三种可能：
①must
②needn't
③变换句式
下面来讲一讲他们分别的用法,一般考试就这三种可能!
1、mustn't→must
mustn't在陈述句中的意思一般是不允许,禁止,所以他的反义疑问句用must或may
例：You mustn't smoke here,must you?或may you?
2、必须→needn't
当must在陈述句中作“必须”解时,它的反义疑问句就变成needn't
例：You must go now ,needn't you?
3、变换句式
在must表示推测,做“一定,准是”,简单的说就是must用于虚拟语气时
像数学一样我们引入一个“常量”——I am sure that
下面就能你那句话作为例子,我们变换一下.
He must have come yesterday.变换句式作
I am sure that he came yesterday.好我们应该分清,反义疑问句问的应该是I am sure that 后面的从句
所以,按照一般反义疑问句的规则得出结果
I am sure that he came yesterday,didn't he?
最后再把句子还原
He must have come yesterday,didn't he?
关键就在于这个题目强调的是or后面的部分,所以用can we?
Open the window,wouldn't/couldn't you?
因为你是祈求的语气,祈使句啊,要用那两个