The boy's father said, “You'll learn what discipline is like when you go into the army. You can't do as you like there, you know, you have to ________.”
句意：男孩的父亲说：“当你参军之后你就会明白什么是纪律。你在军队里不能想干什么干什么，你必须听从命令。”toe the line 听从命令，循规蹈矩。stay in the ring不要放弃。run the race参加赛跑。keep your guard up提高警惕。故答案选A。