must是情態助動詞沒錯, 本句重點在於: must + have Vpp 表示對過去事物的推測, 其中must不是「必須」, 所以附帶問句不可用mustn't; must have been there是過去時態, 如果除去推測語意, 可還原為過去時態的 直述式was there, 所以附帶問句要用wasn't she.
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