Which of the following could be used as a standard for a hotel in determining staffing variances?

a$.$ The average pay rate of housekeeping staff is $$12$ per hour.

b$.$ Both the average pay rate of $$12$ per hour and the average time of $42$ minutes to clean a room could be used as standards.

c$.$ The average time it takes to clean a room is $42$ minutes.

d$.$ The number of guest rooms is $1,000.$