at a time简单

When we look at a face or a picture, our eyes pause on one spot at a time, often on the eyes or mouth.

当我们看一张脸或一幅图片时，我们的眼睛每次停留在一个点上，通常是眼睛或嘴巴。

Buy all your clothes at once with a stylist instead of shopping alone, one article of clothing at a time.

和造型师一起一次性买齐所有衣服，而不是独自一件一件地去买。