Rubidium ${}^{87}_{37}$ Rb Is a Naturally Occurring Isotope That Undergoes β^- Decay

Multiple Choice

Rubidium ${}^{87}_{37}$ Rb is a naturally occurring isotope that undergoes β^- decay.What isotope is the product of this decay?

A) ${}^{87}_{36}$ Kr
B) ${}^{87}_{38}$ Kr
C) ${}^{87}_{37}$ Rb
D) ${}^{87}_{36}$ Sr
E) ${}^{87}_{38}$ Sr