我的皮肤太痒了，我忍不住不停地挠。

My skin was so itchy that I couldn't stop ___scratching.

他在挠蚊子咬的包。

He was ___scratching at his mosquito bites.

她一边思考问题，一边若有所思地挠着头。

She ___scratched her head thoughtfully as she considered the problem.