Please choose the correct ending to complete the given sentence: Pets and Animals: [header] How to treat dental problems in gerbils [title] Look at your gerbil's teeth. [step] Gerbils' teeth continue to grow throughout their lives. To keep their teeth from growing too long, gerbils need to gnaw or chew on something.

Ending1: [substeps] Metal dental dentures need to be fitted to keep the teeth from rusting. The teeth that are fitted in place tend to grow faster. Ending2: Eating right from their dish may help keep this teeth healthy. [substeps] Check the food panel of your gerbil's dish. Ending3: If your gerbil doesn't have much to chew or gnaw on, or is old and cannot chew very well, his teeth will begin to overgrow. [substeps] Sometimes, your gerbil may have broken or missing teeth. Ending4: Gerbils have long, thick, and healthy teeth. [substeps] Some gerbils only eat once a day.

Answer format: ending1/ending2/ending3/ending4