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