1.After that, the number of people in the station plunged to its lowest point of 90 by 4pm
2.The figure then remained stable untill 1984 ai which point the figures began to decline and had dropped to 200 by 2000.
这是我从一篇文章中摘出的2个句子，不知道对不对
请问第一句90后面为什么要用by,用at不行吗，同样是by 像第2句是用了过去完成时，而第一句则是一般时态，是为什么呢