The simplified expression of $(3x^4+x^3-12)+(x^4-x^3+x^2-6)$ is

Question

$\begin{array}{1 1} (a)\;4x^4+x^3+x^2-18 \\(b)\;4x^4+x^3-18 \\ (c)\; 4x^4+x^2-18 \\(d)\; 4x^4+x^2+18 \end{array}$