The amount of a certain radioactive substance decreases by 15% every hour. If a sample starts with 100 g of this radioactive substance, which function represents the amount of radioactive substance left after x hours?

A- f(x)=100(0.15)^x
B- f(x)=100(0.85)^x
C- f(x)=100(1.15)^x
D-f(x)=100(1.85)^x