阅读下面短文，掌握其大意，然后从11－30各题所给的A、B、C和D四个选项中，选出可以填入空白处的最佳选项。
Our heavy lesson
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 1 at the university. More often than not, we 2 him.
One day, we left early, and this time, our uncle had brought us 3. "You can help me carry the 4," he explained.
Sure, 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 he also took some stones out from our bags 5, but 6 he had just found better samples (样品) than the ones we already had.
Still, when we reached the house in the late afternoon, we were 7. The backpacks were so heavy we gave a big sigh of 8 when we put them on the ground. 9 our uncle's backpack was half-empty.
"Why did you give us so 10 stones?" we asked.
"I didn't. You did," he said, waiting for a while before 11. "You did not know it, but I made you 12 a little attitude test today."
"What do you mean an ‘attitude test'?" we wanted to know.
"It is something someone once did to me – and I shall always remember it. You see, I 13 to every word you said. And whenever you 14 about anyone or anything, I added a stone to your bags. And whenever you talked about something with an attitude of 15 and 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 16 thoughts are like stones. You carry them in your mind just like those stones in a backpack. The more negative thoughts you have, the 17 your mind is. A positive thought, however, 18 a negative one. So look at those bags and start paying attention to what you say and how you say things."
This little lesson was one of the most important I 19 had. And I remember my uncle with great love for teaching us one of the most important lessons in life – the 20 of attitude