Jai was measuring the radioactivity of different radioactive isotopes. isotope a had a starting radioactive level of 50 g, and after 4 days it measured at 30 g. isotope b had a starting radioactive level of 75 g, and after 4 days it measured at 25 g. which best explains the isotopes? isotope a has a shorter half-life than isotope b., isotope a has a longer half-life than isotope b., isotope a has half the half-life of isotope b., or isotope a has twice the half-life of isotope b.