Correct Option: C

Natural uranium consists of three major isotopes: U238 (99.28% natural abundance), U235 (0.71%), and U234 (0.0054%). All three are radioactive, emitting alpha particles, with the exception that all three of these isotopes have small probabilities of undergoing spontaneous fission, rather than alpha emission. U238 is usually an alpha emitter (occasionally, it undergoes spontaneous fission), decaying through the “Uranium Series” of nuclear decay, which has 18 members, all of which eventually decay into Pb206, by a variety of different decay paths. The decay series of U235, which is called the actinium series has 15 members, all of which eventually decay into Pb207. The constant rates of decay in these decay series makes the comparison of the ratios of parent to daughter elements useful in radiometric dating.