鱼在游泳时，产生前进动力的部位主要是（ ）
A．尾部的摆动
B．胸鳍和腹鳍的摆动
C．躯干部和尾部的摆动
D．各种鳍的共同作用
鲫鱼的鳍有尾鳍、臀鳍、背鳍、腹鳍、胸鳍．其中胸鳍和腹鳍有保持鱼体左右方向的平衡的作用，胸鳍还有转换方向的作用；背鳍有保持竖直方向的平衡的作用；尾鳍能够产生前进的动力和控制运动的方向．鱼在游泳时，前进的动力来自于躯干部和尾鳍的摆动．各种鳍是相互协作的．
故选：C
Many teenagers feel that the most important people in their lives are their friends. They believe that their family members, especially their parents, don't know them as well as their friends do. In large families, it is often for brothers and s  isters to fight with each other and then they can only go to their friends for ad vice. It is very important for teenagers to have one good friend or many friends. Even when they are not with their friends, they usually spend a lot of time talking among themselves on the phone. This communication is very important in children's growing up, because friends can discuss something difficult to say to their family members. However, pare  nts often try to choose their children's friends for them. Some parents may even stop their children fr om meeting their good friends. The question of "choice" is an interesting one. Have you ever thought of the following questions?Who choose your friends? Do you choose your friends or your friends choose you? Have you got a good friend your parents don't like? 小题1:Many teenagers think their know them better than their parents do.
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