What is the simplified form of the expression 8x^2 + 4xy - 24y^2?
A) 12x^2 - 4xy - 24y^2
B) 8x^2 + 4xy - 24y^2
C) 8x^2 + 4xy + 24y^2
D) 8x^2 - 4xy + 24y^2