7. A radioactive substance decays at an annual rate of 13 percent. If the initial amount of the substance is 325 grams,which of the following functions f models the remaining amount of the substance, in grams, t years later? \text { A } f(t)=325(0.87)^{t} \text { B. } f(t)=325(013)^{c} \text { c } f(t)=0.87(325)^{\mathrm{c}} \text { D. } f(t)=013(325)^{t}

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