My teenage son Karl became  _ after his father died. As a single parent, I tried to do my best to talk to him. But the more I tried, the more he pulled away. When his report card arrived during his junior year, it said that he had been absent 95 times from classes and had six failing grades for the year. In this way he would never graduate. I sent him to the school adviser, and I even begged him. Nothing worked. One night I was at work when I got a phone call. A man introduced himself as the headmaster. "I want to talk to you about Karl's absences." Before he could say another word I choked up  and said sadly, "I love my son. I've tried everything to get Karl to go back to school and nothing has worked. It's out of my hands." For a moment there was silence on the other end of the line. The headmaster seriously said, "Thank you for your time." and hung up. Karl's next report card showed a marked improvement in his grades. He was even on the list of the best students at school. In his fourth year, I attended a parent-teacher meeting. I noticed that his teachers were surprised at the way he had turned himself around. On our way home, he said, "Mum, remember that call from the headmaster last year?" I nodded. "That was me. I thought I'd play a joke but when I heard what you said, it really hit me how much I was hurting you. That's when I knew I had to make you proud. Who was it that telephoned Karl's mother that day?
The headmaster The school adviser Karl himself The passage doesn't tell us