Two sealed samples each contain 1.00×1012 nuclei of the same radioisotope. Sample X is a finely divided powder; sample Y is a single large crystal. Both are at the same temperature. Which statement about their initial activities is most accurate?
(a) Sample X has greater activity because its larger surface area exposes more nuclei to decay.
(b) Sample Y has lower activity because the thick crystal self-absorbs some radiation, slowing decay.
(c) The activities are equal because activity depends only on the number of undecayed nuclei.
(d) Both activities are zero because radioactive decay requires an external trigger that is absent in a sealed container.