Solve the system.

$x_1-3x_3=12$

$2x_1+4x_2+9x_3=-9$

$2x_2+4x_3=-6$

Select the correct choice below and, if necessary, fill in the answer boxes to complete your choice.

A$.$ The unique solution of the system is

$($Type integers or simplified fractions.$)$

B$.$ The system has infinitely many solutions.

C$.$ The system has no solution.