The Forward Rate Constant, K_f, and Reverse Rate Constant, K_r

The forward rate constant, k_f, and reverse rate constant, k_r, for a chemical reaction are not equal. Which of the following must be true?

A) The reaction will be unable to achieve equilibrium.
B) k_f and k_r will become equal as equilibrium is approached, owing to concentration changes.
C) k_f and k_r will become equal as equilibrium is approached, owing to temperature changes.
D) k_f and k_r will remain unequal but the rates will become equal, owing to concentration changes.
E) k_f and k_r will remain unequal but the rates will become equal, owing to temperature changes.