A、Swim straight into the same tank. B、Produce the appropriate sound. C、Raise their heads above the water. D、Press the right-hand

W: One of the most interesting experiments with dolphins must be one done by Doctor Jarvis Bastian. What he tried to do was to teach a male dolphin called Bass and a female called Doris to communicate with each other across a solid barrier.
M: So how did he do it exactly?
W: Well, first of all, he kept the two dolphins together in the same tank and taught them to press levers whenever they saw a light. The levers were fixed to the side of the tank next to each other. If the light flashed on and off several times, the dolphins were supposed to press the left-hand lever followed by the right-hand one. If the light was kept steady, the dolphins were supposed to press the levers in reverse order. Whenever they responded correctly, they were rewarded with fish.
M: Sounds terribly complicated.
W: Well, that was the first stage. In the second stage, Doctor Bastian separated the dolphins into two tanks. They could still hear one another, but they couldn’t actually see each other. The levers and light were set up in exactly the same way except that this time it was only Doris who could see the light indicating which lever to press first. But in order to get their fish, both dolphins had to press the levers in the correct order. This meant of course that Doris had to tell Bass whether it was a flashing light or whether it was a steady light.
M: So did it work?
W: Well, amazingly enough, the dolphins achieved a 100 % success rate.
Questions 19 to 21 are based on the conversation you have just heard.
19. What is the purpose of Doctor Jarvis Haitian’s experiment?
20. What were the dolphins supposed to do when they saw a steady light?
21. How did the second stage of the experiment differ from the first stage?