What Is the H₃O⁺ Concentration in 0 $\circ$ C? (K_w = 1

What is the H₃O⁺ concentration in 0.0042 M Ba(OH) ₂(aq) at 25 ^$\circ$ C? (K_w = 1.01 $\times$ 10^-14) ?

A) 8.4 $\times$ 10^-3 M
B) 4.2 $\times$ 10^-3 M
C) 1.2 $\times$ 10^-12 M
D) 2.4 $\times$ 10^-12 M
E) 1.0 $\times$ 10^-7 M