Please choose the correct ending to complete the given sentence: Pets and Animals: [header] How to keep an older dog mentally active [title] Make sure your dog gets lots of interaction. [step] Dogs that are left alone all the time will not be able to stay mentally healthy. A dog that does not have a lot of social interaction can lose its mental acuity because it doesn't exercise its brain.

Ending1: Dogs are meant to have social interaction, as they are pack animals, and they need it to avoid becoming sad, lonely, and destructive. [substeps] Interaction can come in a lot of forms. Ending2: If you have an older dog, be sure to give it high-quality attention. Maintain a routine that you don't mind ruining when you leave it alone. Ending3: When dealing with an older dog it is important that it gets lots of activity-exercise both inside its mind and its physical. [substeps] If your senior dog is over 10 months old, avoid trying to juggle walks with your aging dog. Ending4: Young dogs can become aggravated by changes in the environment, such as lack of coordination or restlessness. Breeds that are left alone all the time, such as african grey longhaired golden retrievers, have clear brain function.

Answer format: ending1/ending2/ending3/ending4