Write the next sentence in the following story.

How to keep pet rabbits cool
Monitor the temperature.
The ideal temperature for a rabbit is between 55 and 70 degrees fahrenheit. They can tolerate temperatures as high as 85 degrees fahrenheit, if absolutely necessary, but anything higher will increase the risk of heat stroke.

OPTIONS:
[a]. As a result, monitor the temperature once per day and check for signs of heat fluctuation. Some sources of heat fluctuation in rabbit cages include glass or opaque windows..
[b]. If you keep your rabbit outdoors, pay attention to the temperature. Summertime is the most common time of the year to be concerned..
[c]. Temperatures from 54 to 79 degrees fahrenheit are considered to be at the ideal temperature for a rabbit's body heat. To check temperatures properly, hang a thermometer on your wall or window..
[d]. If the temperature is below a certain level, you'll need to get your rabbit's body temperature measured in millimeters instead of body degrees; thus, encourage your rabbit to move around in the coop to keep them cool. If your rabbit spends a significant amount of time outdoors, consider keeping him inside his cage... The answer should be
[b].