Sally broke up with her boyfriend. She refuses to text him back 90% of the time when he texts her. Sometimes, this means she will not text back after 10 or 20 texts. Finally, she gets frustrated (and lonely) and will text back but only to say 'hi,' 'good luck at your football game,' or other seemingly meaningless comments. This process continues for months, where he texts her a few times every day, and eventually she will break down and answer. Which schedule of reinforcement is the reason the boyfriend is still texting so long after the breakup?