Rubidium $\begin{array} { l } 87 \\37\end{array}$ Rb Is a Naturally Occurring Isotope That Undergoes ?^- Decay

Rubidium $\begin{array} { l } 87 \\37\end{array}$ Rb is a naturally occurring isotope that undergoes ?^- decay. What isotope is the product of this decay?

A) $\begin{array} { l } 87 \\36\end{array}$ Kr
B) $\begin{array} { l } 87 \\38\end{array}$ Kr
C) $\begin{array} { l } 87 \\37\end{array}$ Rb
D) $\begin{array} { l } 87 \\36\end{array}$ Sr
E) $\begin{array} { l } 87 \\38\end{array}$ Sr