已知数据 ， ， 的平均数是 ，那么 ， ， 的平均数是（    ）.

阅读理解

    Mr. Smith made many tests with different animals. They found that the monkey was the cleverest of all the animals. One day Mr. Smith put a monkey in a room. He also put some small boxes in it. In one of the boxes there was some food. “How long will it take the monkey to find the food?” Mr. Smith said to himself. “Let me wait and see.” He left the room and waited outside. Three minutes later, he put his eye to the keyhole (猫眼). What did he see? He saw the eye of the monkey. The monkey was on the other side of the door and looked at Mr. Smith through the keyhole.

被子植物的根、茎、叶、花、果实、种子的形态结构都可以作为分类的依据。