单项选择题

What can dogs see The answer to this question is disappointing; dogs (67) see no colors. The answer is disappointing (68) many owners of dogs will naturally be sorry that their dogs cannot see colors which to them are beautiful. But then, they may (69) that clogs have an extraordinary (70) sense of smell. The dog’s world is (71) in enjoyable smells, (72) it may be colorless. How do we know if dogs are color-blind This has been tested in the same way (73) it has been discovered what dogs can hear. The (74) has been made to train dogs to salivate (流口水) when they are shown (75) different colors. Such experiments have (76) failures; it has been found impossible to make dogs (77) colors from one another (78) signals for their dinner. This question requires further (79) with other techniques, but so far as the available scientific evidence (80) , dogs seem to be color-blind. Monkeys, on the other hand, are able to distinguish colors. They have been trained successfully to (81) their meal to a cupboard, the door of (82) was painted in a certain color, and to (83) other available cupboards with differently colored doors, (84) which there was no food. (85) monkeys and apes, (86) , most mammals seem to be color-blind. Even bulls have been shown not to see red as a color.