swear by造句

Many people swear by vitamin C's ability to ward off colds.
许多人深信维生素C能预防感冒。

I swear by God I didn't mean it.
我对上帝发誓我不是这个意思.

I thought I saw him leaving, but I couldn't swear by it.
我好象看见他离开了, 但我不能肯定.