A 0.10 M solution of a weak monoprotic acid has a hydronium-ion concentration of 4.2 times 10^4 M. What is the acid-ionization constant, K_a, for this acid? A) 2.0 times 10^-2 B) 2.9 times 10^-3 C) 4.2 times 10^-4 D) 4.2 times 10^-4 E) 5.0 times 10^-5