阅读下面的短文，掌握其大意，然后从短文后各题所给的 A、B、C、D 四个选项中，选择最佳选项。
When my brother and I were children, we spent a few weeks each summer in the countryside with our uncle. He was a geologist（地质学家）and loved to go on long1 to find stones to add to his collection at the university. More often than not, we2 him.
One day, we left early, and this time, our uncle gave us two 3. "You can help me carry the samples（样本）," he told me.
"Surely, 4" I said. 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 out5 our bags, but we thought he had just found6 samples.
We reached the house in the late afternoon. The backpacks were7 heavy that we were really tired out when we put them on the8 . Yet our uncle's backpack was half-empty.
"Why did you give us so many stones?" we asked.
"I didn't 9 did," he said, waiting for a while before going on. "You did not know it, but I made you go through a little attitude（态度）10 today."
"What do you 11an „attitude test'?" we wanted to know.
"It is something that someone12 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（肯定）thinking, I took out a stone. And now look at your backpacks."
We did. They were 13.
"I hope you remember this little attitude test. You see, your negative（否定）thoughts are like stones. You carry them in your14just like those stones and start to care 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 great15 for teaching us one of the most important lessons in life – the power of attitude.