Used in rhetorical questions to emphasize disbelief or surprise; often used in the patterns 难道...吗 nán​dào... ma or 难道...不成 nán​dào... bù​chéng, meaning "could it be that..." or "is it possible that..."

Used at the end of a sentence to add emphasis to a rhetorical question, often used together with 难道 nán​dào or 莫非 mò​fēi to mean "can it be that...?".