The system of equations in C) x + 6y = -18 and x - 6y = 18 has infinitely many solutions.
To see this, you can solve for x in one of the equations and substitute that expression for x in the other equation. For example, if you solve for x in the second equation, you get:
x = 18 + 6y
Substituting this expression for x in the first equation gives:
(18 + 6y) + 6y = -18
24y = -36
y = -1.5
Substituting this value for y back into the expression for x gives:
x = 18 + 6y = 18 + 6(-1.5) = 18 - 9 = 9
So, one solution to the system of equations is (x, y) = (9, -1.5). However, since there is a variable on both sides of the equation, there are infinitely many solutions to this system of equations.
All of the other systems of equations have either no solutions or exactly one solution.