In scientific notation, what is the sum of 2.3 times 10^{-3} and 5.4 times 10^{-4}?

a. 2.84 times 10^{-3}
b. 5.63 times 10^{-3}
c. 7.7 times 10^{-7}
d. 12.42 times 10^{-12}