他小声嘟囔着道歉。

He muttered an apology ___under ___his ___breath.

她掉手机时小声咒骂。

She cursed ___under ___her ___breath when she dropped her phone.

老师小声责备学生，以免打扰别人。

The teacher scolded the student ___under ___her ___breath to avoid disturbing others.