核心问题
They must be playing basketball, ____?
A. must they B. mustn't they C. are they D. aren't they 《中国高考英语考点50讲》(南大出版社，p.228)给的答案是D，并解答说：“must表示猜测，附加疑问部分要与must之后的主要动词相呼应。” 请说明上题为什么不可以选B。
词典解答
上面的答案和解答是正确的,见 tag questions 之五。但是,根据 PEU 1995 年版上的例句,上题也可以选 B:
It must be nice to be a cat, mustn't it? 做猫一定很好,是不是?再如:
He must have left, mustn't he? 他一定离开了,是不是?
薄冰教授也举过这样的例子：
-
He must have left, mustn't he? [《英语语法札记》(四), p. 288]
-
She must have come to some harm to be out on the loose like she was, mustn't she? [《教你解决语法难点》(一), p. 127]
(参见 tag questions 之五)