Definition: to sing without opening your mouth Level: C2
她在房子里走来走去时哼着一首歌。
She ___hum a song while walking around the house.
秘密地，当没人能听见时，他们哼着调子。
Secretly, they ___hum a tune when nobody can hear them.
他沉思时经常不自觉地哼唱。
He often ___hum without realizing it when he's deep in thought.
