When a new cellphone is put on the market, the demand each month can be described by the function C of t is equal to negative square root of the quantity t squared minus 4 times t minus 12 end quantity plus 3 where C (t) represents the demand of the cellphone (measured in millions of people) and the time, t, is measured in months. Which of the following solution(s) are valid for a positive demand?

(2, 3)
(6, 3)
(3, 0) and (7, 0)
(–2, 3) and (6, 3)