we finished all of the food at a sitting . the underlined part means 

a. at one time 

b.in time 

c.on time 

d.at a time

at a sitting 一口气地，一下子地

Gallants gambled away a fortune at a sitting.  

时髦男士们一次赌博就要输掉一大笔钱.

The job was finished at a sitting .  

这工作一下子就做完了.

. He read the novel at a sitting.  

他一次把那本小说读完.

http://ask.yygrammar.com/q-29251.html

(1)at a time = at a single time 每次、每一次、 

Pass me the bricks two at a time. 

一次递给我两块砖。

He spoke two hours at a time. 

他每次都讲了两个小时的话。

Add the cream a few drops at a time.  

每次加几滴奶油。

(2)at one time = at an unspecified time in the past 过去有个时候、曾经，一下子

At one time I used to go swimming every Sunday. 

有一个时期，我每个星期天都去游泳。

It is said that at one time he lived apart from his wife. 

据说，他曾和妻子分居。

.My brain can't hold so much information at one time.  

我脑子一下子记不住这麽多信息.