The amount of radioactive substance left at the end of x days can be represented by an exponential function. the initial amount is 100 grams, and the amount left after three days is 72.9 grams. which function models the amount of radioactive substance left at the end of x number of days? a. f(x)=0.85^x b. f(x)=0.90^x c. f(x)=100(0.85)^x d. f(x)=100(0.90)^x steps with the i was never taught