New experiments discover rats showing sympathy and helping other rodents  .It is a characteristic some scientists thought was reserved only for humans and higher animals. In repeated tests, rats freed another trapped rat in their cage, even when yummy chocolate served as an attraction.24 of the 30 rats opened the trap by pushing in a door.The rats could have eaten the chocolate before freeing their partners, but often didn't, choosing to help and share the goodies. "Basically they told us freeing another rat is as important as eating chocolate," said study author Peggy Mason of the University of Chicago."That's a very striking thing." In some cases, the rats first took the chocolate chips out of a container, but didn't eat them, then freed the other rat and shared "almost as if they were serving them chocolate," Mason said.The research is reported in Thursday's journal Science. Also, females showed more consistent care and help than males, Mason said.All six females freed their trapped partner while three quarters of the 24 males did so.This confirms other studies that show females showing more pro-social   behavior than males.There were time when the male rats took the day off from helping their trapped partner, but the females never did, she said. Jeff Mogil at McGill University in Canada, who wasn't part of the study, said it was a little surprising but even more convincing. Both scientists said social understanding is probably a characteristic that is important in the evolution of animals. Mason joked that if rats can be so caring and helpful, "there's a sense of optimism.It's something we could be." In the experiments,   _   male rats freed their trapped partners.
6 18 24 30
Question: New experiments discover rats showing sympathy and helping other rodents  .It is a characteristic some scientists thought was reserved only for humans and higher animals. In repeated tests, rats freed another trapped rat in their cage, even when yummy chocolate served as an attraction.24 of the 30 rats opened the trap by pushing in a door.The rats could have eaten the chocolate before freeing their partners, but often didn't, choosing to help and share the goodies. "Basically they told us freeing another rat is as important as eating chocolate," said study author Peggy Mason of the University of Chicago."That's a very striking thing." In some cases, the rats first took the chocolate chips out of a container, but didn't eat them, then freed the other rat and shared "almost as if they were serving them chocolate," Mason said.The research is reported in Thursday's journal Science. Also, females showed more consistent care and help than males, Mason said.All six females freed their trapped partner while three quarters of the 24 males did so.This confirms other studies that show females showing more pro-social   behavior than males.There were time when the male rats took the day off from helping their trapped partner, but the females never did, she said. Jeff Mogil at McGill University in Canada, who wasn't part of the study, said it was a little surprising but even more convincing. Both scientists said social understanding is probably a characteristic that is important in the evolution of animals. Mason joked that if rats can be so caring and helpful, "there's a sense of optimism.It's something we could be." In the experiments,   _   male rats freed their trapped partners.
A. 6
B. 18
C. 24
D. 30
Answer: