One day, a poor man, who had only one piece of bread to eat, was walking past a restaurant. There was a large pot of soup on the table. The poor man held his bread over the soup, so the steam from the soup went into the bread, and gave it a good smell. Then he ate the bread.

    The restaurant owner was very angry at this, and he asked the man for money, in exchange for the steam from the soup. The poor man had no money, so the restaurant owner took him to Nasreddin, who was a judge at that time. Nasreddin thought about the case for a little while.

    Then he took some money from his pocket. He held the coins next to the restaurant owner's ear, and shook them, so that they made a dingling (叮当声) noise.

    “What was that?” asked the restaurant owner.

    “That was payment for you,” answered Nasreddin.

    “What do you mean? That was just the sound of coins!” said angrily the restaurant owner.

    “The sound of the coin is payment for the smell of the soup,” answered Nasreddin. “Now go back to your restaurant.”

化简（ab+b²）÷$\frac{a^2-b^2}{a}$的结果是（　　）

进行实验方察设计时，为了排除其他因素的干扰，使影响实验效果的因素能准确明显地表现出来，一般只考虑（　　）

木工师傅在做完门框后，为防止变形常常像图中所示那样钉上两条斜拉的木板条（即图中的AB和CD），这样做的根据是（　　）