我在鞋子里发现了一粒沙子。

I found a ___grain of sand in my shoe.

她又心不在焉地笑了笑，从钢琴盖上擦掉一粒灰尘。

She smiled again, absentmindedly, and wiped away a ___grain of dust from the lid of the piano.

没有任何证据支持他的说法。

There isn't a ___grain of evidence to support his claim.