My teenage son Karl became withdrawn after his father died.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 falling grades for the year. In this case he would never graduate. I sent him to the school adviser, and I even begged（乞求）him. Nothing worked.
One night I felt so powerless that I got down on my knees and begged for help. "Please God, I can't do anything more for my son.  I'm at the end of my rope. I'm giving the whole thing up to you."
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（哽咽） up and all my disappointment（失望） and sadness over Karl came pouring out into the ears of this stranger. "I love my son but I just don't know what to do. 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. Finally, he was even on the list of the best students at school. In his fourth year, I attended a parent-teacher meeting with Karl. I noticed that his teachers were surprised at the way he had turned himself around.
After the meeting, Karl 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."
By saying "Karl became withdrawn", the writer means the boy changed completely and _______.

A．preferred to stay alone at home

B．lost interest in his studies

C．refused to talk to others

D．began to dislike his mother

There was silence on the other end of the line because the speaker _______.

A．was too moved to say anything to the mother

B．waited for the mother to finish speaking

C．didn't want the mother to know who he was

D．didn't want to say to his mother

The phrase "at the end of my rope" means _______ in Chinese.

A．心如刀割

B．心意冷

C．失魂落魄

D．山穷水尽

What does this passage mainly tell us?

A．Being understood by parents is very important to teenagers.

B．Mother's love plays an important part in teenagers' life.

C．School education doesn't work without full support from parents.

D．Children in single-parent families often have problems with their mind.

The best title for the passage is _______.

A．A lonely boy

B．How to get on with a teenager

C．A student who turned himself around

D．A joking phone call