频繁使用这把剪刀在硬纸板上会很快使它变钝。

Using these scissors on cardboard frequently is ___dulling them quickly.

长时间的讲座让学生的注意力变得迟钝。

The long lecture is ___dulling the students' attention.

这些止疼药正在减轻她的头痛。

The painkillers are ___dulling her headache.