Each year, the amount of the radioactive substance is reduced by 13 percent from the prior year’s amount; that is, each year, 87 percent of the previous years amount remains.Since the initial amount of the radio active substance was 325 grams,after 1 year .325(0.87) grams remains;after 2 years 325(0.87)(0.87)=325(0.84)2 grams remains;and after t years,325(0.87)t grams remains.Therefore ,the function f(t)=325(0.87)t models the renaming amount of the substance,in grams after t years Choice B is incorrect and may result from confusing the amount of the substance remaining with the decay rate. Choices C and D are incorrect and may result from confusing the original amount of the substance and the decay rate.