The Reaction a → B Follows First-Order Kinetics with a Half-Life

The reaction A → B follows first-order kinetics with a half-life of 21.7 hours.If the concentration of A is 0.023 M after 48.0 hours,what was the initial concentration of A?

A) 0.0050 M
B) 0.051 M
C) 0.51 M
D) 0.11 M
E) 2.0 M