她把一缕头发别到耳后。

She tucked a ___strand of hair behind her ear.

她注意到一缕松散的白发，立刻把它别了回去。

She noticed a loose ___strand of white hair and immediately pinned it back.

绳子的一股线磨损了，削弱了整个结构。

A single ___strand of the rope had frayed, weakening the whole structure.