The girl would,---go to visit the great scientist on his way

The girl would,___go to visit the great scientist on his way back home after school.A.at a time B.at the timeC.at the same timeD.at one time请问at a time ,at one time 这题应该选什么?另外做到过一片完型：By the time the Lord made the first mother,he was into the sixth day working overtime.An Angel appeared and said,“Why are you spending so much time on this one!”And the Lord answered and siad,“Have you read the spec sheet on her!She has to be completely washable,but not elastic have 200 movable parts,all replaceable run on black coffee and leftovers have a lap that can hold three children at 【one】 time and that disappears when she stands up have a kiss that can cure anything from a scraped knee to a broken heart and have six pairs of hands.”这篇文章要表达的应该是一次、每次呀,为什么不是 at a time?答案上说at a time 是“一度,曾经” at one time 是“一次,同时”很疑惑~但是网上好像这篇文章都是用的 at one time刚查了牛津高阶,确实at a time=每次,逐一at one time=曾经那篇文章不通顺啊那!