Look, this is my city. There are many bikes and buses on the streets. Let's count. There are ____  bikes and ____  buses. Look here, there are also ____   taxis and ____  cars. My city is big and beautiful!

    A man was traveling abroad in a small red car. One day he left the car and went shopping. When he came back, its roof was badly damaged.

    Some boys told him that an elephant had damaged it. The man did not believe them, but they took him to a circus(马戏团) which was near there. The owner of the elephant said, “I am very sorry! My elephant has a big, round, red chair. He thought that your car was his chair, and he sat on it!"

    Then he gave the man a letter, in which he said that he was sorry and that he would pay for all the damage.

    When the man got back to his own country, the customs（海关） officers wouldn't believe his story. They said, “You sold your new car while you were abroad（国外） and bought this old one!"

    It was only when the man showed them the letter from the circus man that believed him.

①“测量物体的长度”时，多次测量

②“研究杠杆的平衡”时，改变动力（臂）和阻力（臂），多次测量

③“研究重力大小”时，换用不同质量的物体，多次测量

④“用电压表和电流表测导体的电阻”时，多次测量电阻两端电压和通过电阻的电流值

上述实验中目的是为了减小误差的是（   ）