According to the definition of probability density function of two-dimensional random variables and the meaning of classical probability, and including elliptic domain in circular domain, we can know the area of p= elliptic domain "x2+9Y2 ≤ 9A2"/circular domain "x 2+y 2 ≤ 9a 2".

However, the area of an ellipse "x2+9Y2 = 9A2" = π (3a) a = 3π A2, and the area of a circle "x2+Y2 = 9A2" = π (3a) 2 = 9π A2.

∴p=(3πa^2)/(9πa^2)= 1/3。 Therefore, choose B. as a reference.