Which number is the smallest?
A1.28 x 10 ^-4
B2.85 x 10 ^-6
C3.24 x 10 ^-5
D5.48 x 10 ^-8
A1.28 x 10 ^-4
B2.85 x 10 ^-6
C3.24 x 10 ^-5
D5.48 x 10 ^-8