Theo factored a greatest common factor (GCF) of 4x^2 y from the expression 12x^2 y^3 - 20x^3 y^2 + 32x^4 y. What error did Theo make, if any?
a. Theo could have factored an additional x variable from the expression
b. Theo could have factored an additional y variable from the expression
c. Theo didn't make an error. He factored the correct GCF
d. Theo could have factored a higher coefficient from the expression