We know that dogs are a human's best friend. They love us and we love them __________. Many scientists think they came from wolves 15,000 years ago. Although wolves are wild and dogs are tame, they're still a lot alike. Both wag their tails when happy and put their tails between their legs when scared.

• A. However, we're not so sure where they came from
• B. Some were trained to do more than keep us company
• C. Working dogs guide blind and wheel chair bound people
• D. So they're good at finding things using their keen senses