Dolphins are interesting animals. Working with them is fun, because you never know what they will come up with to surprise you.
Remember when your mom used to cut up your food？ Circe, a show dolphin, won't eat fish unless someone cuts the fins (鱼鳍) off. Diana Reiss, a scientist who worked with Circe, knew that. But one day she forgot.
Reiss was training Circe to "stay at station" in front of her. Every time the dolphin swam off, Reiss gave her a time-out by backing away from the side of the pool and standing at a distance. When Circe stayed, Reiss fed her a piece of fish. But then she gave her a piece with fins by mistake.
Circe looked at Reiss, spat(吐) out the fish, and swam across the pool. She took an upright position in the water and simple "stood" there. Circe was communicating (交流) loud and clear. "I felt as if I had gotten the time-out! " says Reiss.
Another dolphin, Pete, enjoyed his meals of fish served up by his trainer. But a wild bird started to show up at every feeding to steal some of the dolphin's supper. Pete was definitely not happy about that.
One day when Pete's trainer gave him frozen fish, Pete let them sink to the bottom of his pool. Between shows he dived down and pushed eight fish up to the surface, and managed to keep all of them afloat. For five minutes, the bird watched anxiously. Finally, the bird took the bait. When it did, Pete caught the bird's feet and pulled it underwater! Then took care of that little thief.
Reiss would feed Circe a fish ______.

A. before Circe gave a performance
B. when Circe followed the instruction
C. the moment Circe swam close to Reiss
D. each time Reiss left the side of the pool