阅读下面短文，用括号内所给动词的适当形式填空(必要时可加情态动词或助动词)
A farmer had some dogs to sell. He painted a sign advertising the dogs. A little boy came up to him and said, “Sir, I want to buy a dog.”
“Well,” the farmer said, “but they_____________________ (be) very expensive.” The boy _____________________ (take) some changes out of his pocket, “I’ve got thirty-nine cents. Is that enough to take a dog?”
“Sure. I have six dogs. You ________________________ (choose) any of them.” Then the farmer whistled, “Dolly!” At once, Dolly ran out of the doghouse followed by four little dogs. Then the little boy’s eyes____________________________ (dance) with happiness.
But the boy saw another little dog___________________________ (move) out slowly. This one was the smallest. She was trying her best _________________________ (catch) up with the others…
“I went that one,” the boy said, ____________________________ (point) to the last one. The farmer was surprised. “She _________________________ (not run) and play with you like other dogs. Will you still want to buy her?” he said kindly.
Hearing that, the little boy rolled(卷起) up one leg of his trousers. A steel brace(支架) _____________________ (tie) on both sides of his leg. Looking up at the farmer, he said, “So you see, Sir. I can’t walk very well myself, I ____________________________ (understand) her. So we will be good friends.”