An expression with at least two terms that share the greatest common factor of 3x⁴y⁶.
a) 3x⁴y⁶ + 2x³y⁵
b) 6x⁴y⁶ + 3x³y⁶
c) 3x⁴y⁶ + 3x²y⁴
d) 3x⁵y⁶ + 2x⁴y⁶