When a new cellphone is put on the market, the demand each month can be described by the function [tex]C(t) = -\sqrt{t^{2} } +2t-15+3[/tex], 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?

a
(5, 3)

b
(3, 3)

c
(6, 0) and (4, 0)

d
(–5, 3) and (3, 3)