Virginia starts with 200 milligrams of a radioactive substance. the amount of the substance decreases by 25% each week for a number of weeks, w. the expression 200(1−0.25)w finds the amount of radioactive substance remaining after w weeks. which statement about this expression is true? a: it is the product of the initial amount and the decay factor after w weeks. b: it is the difference between the initial amount and the percent decrease. c: it is the difference between the initial amount and the decay factor after w weeks. d: it is the initial amount raised to the decay factor after w weeks.