The area of a triangle is $18\;cm^2$ and the sum of its base and altitude is $12$ cm.The measurements of the base and altitude is _____

Question

$\begin{array}{1 1}(A) \;\text{6 cm,5 cm}\\(B)\;\text{6 cm,6 cm} \\(C)\;\text{6 cm,7 cm} \\(D)\;\text{4 cm,4 cm} \end{array}$