First of all, let's determine the distance of the circular track:

Circumference = pi*d = 20*pi

Now, let's represent each car's distance from the starting point (along the track), t hours from when Car A starts:

B(t) = 20 + 2t [Distance traveled after 10 hours, + 2mph)
A(t) = 20*pi - 3t [Starting at starting point (20pi), -3mph)

We need to determine the time it takes for car A to be 12 miles past car B.

B(t) - A(t) = 12
20 + 2t - (20*pi -3t) = 12
t = (-8 + 20pi)/5
t = 4pi - 1.6

Therefore, car A has been traveling (4pi - 1.6) hours before the criterion is satisfied.The question, however, asks how long car B has been traveling.

t + 10 = 4pi - 1.6 + 10
= 4pi + 8.4

Therefore the answer is B: 4pi + 8.4.