The two circles $x^2+y^2=ax$ and $x^2+y^2=c^2\; (c > 0) $ touch each other if

Question

$\begin{array}{1 1}(A)\;|a|=c \\(B)\;a=2c \\(C)\;|a|=2c \\(D)\;2|a|=c \end{array}$