阅读下列短文,从每题所给的四个选项（A、B、C和D）中选出最佳选项。
When my brother and I were children, we spent a few weeks each summer in the countryside with our uncle. He was a geologist and loved to go on long walks to find stones to add to his collection at the university. More often than not, we followed him.
One day, we left early, and this time, our uncle gave us two backpacks. “ You can help me carry the samples(样本),” he explained.
Surely, why not? The whole day, we walked around the countryside, and every now and then, our uncle put stones in our bags. We were a bit surprised that sometimes he also took some stones out from our bags, but thought he had just found better samples.
We reached the house in the late afternoon. The backpacks were so heavy that we were really tired out when we put them on the ground. Yet our uncle's backpack was half-empty.
“Why did you give us so many stones?” we asked.
“I didn't. You did,” he said, waiting for a while before continuing. “ You did not know it, but I made you go through a little attitude(态度) test today.”
“What do you mean an ‘attitude test'?” we wanted to know.
“It is something someone once did to me. You see, I listened to every word you said. And whenever you complained(抱怨) about anyone or anything, I added a stone to your bags. And whenever you talked about something in an attitude of positive thinking, I took out a stone. And now look at your backpacks.”
We did. They were full.
“I hope you remember this little attitude test. You see, your negative thoughts are like stones. You carry them in your mind just like those stones and start paying attention to what you say and how you say things.”
This little lesson was one of the most important lessons I ever had. And I remember my uncle with great love for teaching us one of the most important lessons in life – the power of attitude.
