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.