无人机正在送货上门。
A ___drone is delivering a package.
老师在课堂上用单调的声音讲课，学生们都快睡着了。
The teacher ___droned on in a flat voice, making the students almost fall asleep.
蜜蜂的嗡嗡声在花园里持续不断。
The ___drones of bees filled the garden with a steady hum.
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