53.依半衰期長短排列，下列何者正確？
(A)Ra-226＞Rn-222＞Co-60＞Cs-137
(B)Ra-226＞Co-60＞Rn-222＞Cs-137
(C)Ra-226＞Cs-137＞Rn-222＞Co-60
(D)Ra-226＞Cs-137＞Co-60＞Rn-222