听力原文： When you have a dog it is necessary to care for it carefully. In return you will

The most important part of dog-care is to keep the dog clean. Not only does the dog have to be kept clean, his environment should also be clean. The bed he sleeps in can be made with anything like a rug or a dog mattress.

Proper feeding is also important. A dog should be fed in the same place and on a regular schedule. During feeding time it should be quiet, ff your dog refuses to eat, don't force him. Remove his food and don't feed him again until his next feeding.

Food should never be left before a dog for over thirty minutes under any circumstances, If he does not eat over a long period of time, your dog should be taken to a vet. Another important thing is to keep fresh water available to the dog during hot weather.

One important thing is exercise for dogs. Vigorous exercise helps to build strength and keeps dogs healthy. A regular walk at a fixed time of the day is enough for smaller dogs. Large dogs, though, require a run in an open field.

How long should food be left before a dog？

A．As long as possible,

B．At least 30 minutes.

C．All day.

D．No more than 30 minutes.