Rewrite 18x4y3 − 36x3y4 using a common factor.

a) 2x4y4(9 − 18x)
b) 2x3y3(9y − 18x)
c) 9x3y3(2x − 4y)
d) 9x4y3(2 − 4y)

The correct answer is: 18x4y3 - 36x3y4 = 18x3y3(2x - 4y) = 9x3y3(2x - 4y)