Rewrite 20a4b + 15ab3 using a common factor.

a) 2a4b(10 + 15ab3)
b) 5ab(4a3 + 8ab3)
c) 10ab(2a4b + ab3)
d) 5ab(4a3 + 3b2)

The correct answer is b) 5ab(4a3 + 8ab3).