Write the next sentence in the following story.

How to use an automated litter box
Check instructions for age requirements.
Some self-cleaning litter boxes are only safe for cats of a certain age. Often, your kitten must be at least six months old.

(a). Check the instructions on your model to see if the litter box is safe for your cat. Automatic litter boxes can also be dangerous for extremely elderly or handicapped cats that would have trouble getting in and out of the box. (b). But, some owners limit their own litter box to at least 9 months for their cats. Meeting age requirements cannot only be frustrating for your kitten, but also may end up causing back problems. (c). If the litter box request is for a slight older cat (less than six weeks), then you should not use the litter box for that age group. Be sure to talk to your vet about which litter boxes are safe for this cat. (d). If your cat is so young that it is able to run around the house and ignore you or other household members, the litter box must be safe for that age. You must understand that this is the age required for a litter box sitter to complete the job.. The answer should be
(a).