Mark is solving the following system. Step 1: He multiplies equation (1) by 7 and adds it to equation (3). Step 2: He multiplies equation (3) by 2 and adds it to equation (2). Which statement explains Mark’s mistake?
A. He added equation (3) instead of equation (2) in step 1.
B. He did not multiply equation (3) by the same number as equation (1).
C. He did not eliminate the same variables in steps 1 and 2.
D. He added equation the equations in step instead of subtracting them.