Answer:
D. 12 hours
Explanation:
I am guessing that they want you to find the average time together, or the mean.
Add 18 hours and 6 hours together to get 24 hours.
To get the average, you have to divide the total hours by the amount people involved.
Since there are 2 groups (2 people) that have an outcome of 18 hours and 6 hours, 24 hours/2 people is a 12 hour average.