Hi Pradeep,
Time is not the case, time is a variable. If you look at the table, time is varying and it takes the values 9:00 A.M., 9:30 A.M., and so on.
After every 30 minutes starting from 9:00 AM, the temperature and relative humidity in a room have been recorded. So, 9:00 AM, 9:30 AM, 10 AM,... are the cases because variables are recorded at these times.
Hope this clears your doubt. You can also refer to Lec 1.2.
Thanks and regards
Nikita Kumari
Content Support team