Scientists   think that many animals cannot see colours. The world is black and white to them. Scientists want to find out if   dogs can see colours. When the scientists give the dogs food, they show them a certain colour every time. The dogs get ready to eat when they see this colour. The scientists then show the dogs another colour. Again the dogs get ready to eat. The colours are all the same to them. Colours are not all the same to monkeys. For example, if we put food in a red box, and do this every day, a monkey will always go to the red box to get food. If we put food in a blue box, the monkey won't go to it. The dogs get ready to eat when they   _  .
hear different pieces of music see the monkeys see black things see all the colours