The Indicator Propyl Red Has K_a = 3 $\times$ 10^-6

Multiple Choice

The indicator propyl red has K_a = 3.3 $\times$ 10^-6.What would be the approximate pH range over which it would change color?

A) 3.5-5.5
B) 4.5-6.5
C) 5.5-7.5
D) 6.5-8.5
E) None of these choices is correct.