at a time/at one time

选 B 是错误的, 应当选 A。

at a time 的意思是: “一次” “一下子”:

1. You can borrow only two books at a time. （你）一次只能借两本书。

2. He ran up the steps two at a time. 他一步两个梯级地往上跑。

3. Here are five points that help me learn — and remember — whole families of words at a time. 这里有五点要领可帮助我一下子学到——并记住——词的整个词族。

4. A whale may eat a ton of them at a time, and may enjoy four meals a day. 一头鲸鱼一次就可以吞食一吨这样的小鱼，一天可能要吃四餐。

at one time 的意思是: “一度” “过去曾经” “过去有个时期”:

1. At one time I lived in Japan. 我曾一度住在日本。

2. At one time most school teachers were men, but today there are more women than men. 以前中小学教员大都是男的，如今女的反比男的多了。