Given two fixed points Aand B and AB = 6, then the simplest form of the equation to the locus of P such that PA + PB = 8 is:
a. x₂/12 + y₂/21 = 1
b. x₂/9 + y₂/16 = 1
c. x/16 + y/7 = -1
d. x/16 + y.9=1