Which systems below would be most easily solved using the substitution method?

A. 5x + 7y = 45
2x - 7y = -14

B. 2m - 3n = -2
m + 4n = 24

C. 2x = -3y + 4
x = 2y - 5

D. 7x + 2y = 16
-21x - 6y =24