In a First Order Reaction, the Initial Concentration of a Is

In a first order reaction, the initial concentration of A is 0.40 M. What is the concentration of A after 15 seconds if the half-life of the reaction is 3 seconds?

A) 8.0 $\times$ 10^-2 M
B) 2.0 M
C) 5.0 $\times$ 10^-2 M
D) 2.5 $\times$ 10^-2 M
E) 1.3 $\times$ 10^-2 M