Select all the systems of equations that have exactly one solution.
A. y = 3x + 1 y =− 3x + 7
B. y = 3x + 1 y = x + 1
C. y = 3x + 1 y = 3x + 7
D. x + y = 10 2x + 2y = 20
E. x + y = 10 x + y = 12