A particle moves along the parabolic path $y=a x^{2}$ in such a way that the $x$ component of the velocity remains constant, say $c$. The acceleration of the particle is
(A) $a c \hat{k}$
(B) $2 a c^{2} \hat{j}$
(C) $2 a c^{2} \hat{k}$
(D) $a^{2} c \hat{j}$