19. Rubidium–87 decays by beta decay with a half-life of 4.9×1010years. How many 87Rb atoms are in amoon rock sample that has a rubidium decay rate of 3500 disintegrations per hour?
(A)9.0×1016 atoms
(B)4.3×10⁻⁴ atoms
(C) 2.2×1018 atoms
(D)2.5×1014 atoms
(E)2.6×1015 atoms