英语语法小题目One morning my patience was growing thin__Mark talked

英语语法小题目
One morning my patience was growing thin__Mark talked once too often,and then i made a novice-teacher's mistake.
A.when B.as C.while,D whenever
答案给的是A,但是我选择的是B,我知道C和D是错误的,但是不明白B和A在这里存在什么样的差别呢?

All the Good Things
He was in the first third-grade class I taught at Saint Mary's School in Morris,Minnesota.All 34 of my students were dear to me,but Mark Eklund was one in a million[1].Very neat in appearance,he had that happy-to-be-alive attitude[2] that made even his occasional mischievousness delightful.
Mark also talked incessantly.I had to remind him again and again that talking without permission was not acceptable.What impressed me so much,though,was his sincere response every time I had to correct him for misbehaving."Thank you for correcting me,Sister[3]!" I didn't know what to make of it at first,but before long I became accustomed to hearing it many times a day.
One morning my patience was growing thin[4] when Mark talked once too often,and then I made a novice-teacher's mistake.I looked at Mark and said,"If you say one more word,I am going to tape your mouth shut[5]!"
答：此题句摘自新编大学英语阅读部分第三册Unit05-2 ,来自《心灵鸡汤》,上文供参考.
记住：sb.was doing sth.when sth.happened.这个句型能使你做题又快又对.
译：有一天上午,我的耐心将至极限,就在这时,Mark又说话了,这次我是忍无可忍了.我犯了一个新老师才会犯的错误：.
再问： 查询了一下，once too often有又一次，和许多次的含义，所以我弄不清楚这里是使用了哪个含义，根据句子的前半部分，老师的耐心渐减是一个过程，所以后面不应该是许多次造成的吗？ 那样看得话应该是使用as呀，至少when与as都是正确的，你说呢？
再答： too often是经常的意思。once too often,意为再来一次，就太多了。就是已经到了忍无可忍的地步了。 语言有个习惯的问题。没有绝对的正确与不正确。大家都认为这么说好，这就是语法：语言的规律。