What can dogs see? The answer to this question is disappointing dogs【C1】______see no colors. The answer is disappointing【C2】______many owners of dogs will naturally be sorry that their dogs cannot see colors which to them are beautiful. But then, they may【C3】______that clogs have an extraordinary【C4】______sense of smell. The dog's world is【C5】______in enjoyable smells,【C6】______it may be colorless. How do we know if dogs are color-blind? This has been tested in the same way【C7】______it has been discovered what dogs can hear. The【C8】______has been made to train dogs to salivate(流口水)when they are shown【C9】______different colors. Such experiments have【C10】______failures it has been found impossible to make dogs【C11】______colors from one another【C12】______signals for their dinner. This question requires further【C13】______with other techniques, but so far as the available scientific evidence【C14】______, dogs seem to be color-blind. Monkeys, on the other hand, are able to distinguish colors. They have been trained successfully to【C15】______their meal to a cupboard, the door of【C16】______was painted in a certain color, and to【C17】______other available cupboards with differently colored doors,【C18】______which there was no food.【C19】______monkeys and apes,【C20】______, most mammals seem to be color-blind. Even bulls have been shown not to see red as a color. 【C1】