A．how to write a thank-you letterB．how to deal with car……

Many years ago, when I was fresh out of school and working in Denver, I was driving to my parents’ home in Missouri for Christmas. I stopped at a gas station(加油站) about 50 miles from Oklahoma City, where I was planning to stop and visit a friend. While I was standing inline at the cash register, I said hello to an older couple who were also paying for gas.
I took off, but had gone only a few miles when black smoke poured from the back of my car. I stopped and wondered what I should do. A car pulled up behind me. It was the couple I had spoken to at the gas station. They said they would take me to my friend’s home. We chatted on the way into the city, and when I got out of the car, the husband gave me his business card.
I wrote him and his wife a thank-you note for helping me. Soon afterward, I received a Christmas present from them. Their note that came with it said that helping me had made their holidays meaningful.
Years later, I drove to a meeting in a nearby town in the morning. In late afternoon I returned to my car and found that I’d left the lights on all day, and the battery was dead. Then I noticed that the Friendly Ford dealership—a shop selling cars was right next door. I walked over and found two salesmen in the showroom.
"Just how friendly is Friendly Ford" I asked and explained my trouble. They quickly drove a pickup truck to my car and started it. They would accept no payment, so when I got home, I wrote them a note to say thanks. I received a letter back from one of the salesmen. No one had ever taken the time to write him and say thank you, and it meant a lot, he said.
"Thank you" are two powerful words. They’re easy to say and mean so much.
By telling his own experiences, the author tries to show ______.

A．how to write a thank-you letter
B．how to deal with car problems
C．the kind-heartedness of older people
D．the importance of expressing thanks