根据短文理解，从A、B、C、D四个选项中选择最佳答案填空。
An old man was in a bank. He was arguing 1 how much money he had in the bank. 2 he was arguing, a young man came into the bank and said to the old man, "I've just come from your son. He gave me a letter and this piece of silver for you."
He then gave the silver to the old man. Because the old man had weak eyes-he asked the banker to read him his son's letter.
The banker read," I have 3 you a piece of silver that weighs 4 kilos."
The old man was very 4 and asked the banker to give him some dollars for his silver. But when the banker weighed the silver, he 5 that it weighed 5 kilos.
"The old man will never know," he thought. So he only gave him dollars for 4 kilos of silver.
Later the banker examined the silver 6 and discovered that it was not silver. It was tin(锡). 7 he ran after the old man and shouted, "You have 8 me. You gave me tin, 9 silver."
The old man then asked the banker to weigh the piece of tin. It weighed 5 kilos.
"I am not robbing you, "said the old man. "You are trying to rob me! The piece of silver I gave you 10 weighed 4 kilos!"