Given the following system, what would be the best way to start solving the system? x+y=20
x=3y
(1 point)
A Solve for y in the second equation, and substitute x3 for y in the first equation.
B Solve for y in the first equation, and substitute the solution for y in the second equation.
C Substitute 3y from the second equation for x in the first equation.
D Solve for x in the first equation, and substitute the solution for x in the second equation.
I think D (please explain why don't just give the answer)