This chart show the amount of radioactivity measured from three unknown isotopes.

which statement describes each of the isotopes?
isotope a was measured at day 24, isotope b was measured at day 6, and isotope c was measured at day 10.
isotope a was measured at day 18, isotope b was measured at day 4, and isotope c was measured at day 5.
isotope a was measured at day 24, isotope b was measured at day 8, and isotope c was measured at day 20.
isotope a was measured at day 30, isotope b was measured at day 6, and isotope c was measured at day 10.