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Which are true when solving this system by substitution?
y = 2x + 2
4x + 7y = 32

a. The solution is (1,4).
b. The variable x is the one to be isolated.
c. 4(2x + 2) + 7(2x + 2) = 32 is the correct substitution.
d. The first equation is already solved for y.