6. Look at the four squares, A , B , C and D , which indicate where the following sentence could be added to the passage. Where would the sentence best fit?
The front end of the body bends less than the back end, so each undulation increases in size as it travels along the body.
We can understand how fish swim by studying the motion of a very flexible fish such as an eel. A The movement is serpentine, with undulations moving backward along the body by alternate contraction of the muscles on either side of the eel’s body. B While the undulations move backward, the bending of the body pushes sideways against the water, producing a reactive force that is directed forward at an angle. C The movement has two components: thrust and lateral force. D Thrust is used to propel the fish forward, and lateral force tends to make the fish’s head deviate from the course in the same direction as the tail. This side–to–side head movement is very obvious in a swimming eel, but fish with large, rigid heads have enough surface resistance to minimize the lateral movement.