题目

Wild animals can take care of themselves．But __1__ need special care．You can loam much about animals __2__ taking care of a pet．Owning a pet is almost like __3__ a patient．You must think about __4__ your pet needs．You have to give it food and water and a safe home．You must teach your pet and love your pet．__5_ You do all these things well，you will have a healthy pet． Some people like dogs best．They give them food and water and a __6__ to sleep in．Dogs love to take walks．Most dogs can learn tricks（戏法），like fetching a ball or rolling over and so on． Most child设变量x，y满足约束条件：则z=x-3y的最小值A.-2B.-4C.-6D.-8