so与so that摘自一道完形填空文章讲的是如何教育孩子吃饭...,and do not agree him(指孩子)

so与so that
摘自一道完形填空
文章讲的是如何教育孩子吃饭
...,and do not agree him(指孩子) to leave the table immediatly after a meal or he will soon learn to swallow his food ________ he can hurry back to his toys.
A.so
B.until
C.lest
D.although
参考答案是A,但我选的是B,也不知道为什么选A.
感觉在横线上填上 so that 最好不过了.
1.为什么选A?
2.填 so that 可不可以?