When Testing the Hypothesis Presented Below, at a $\begin{array}{l}H_{0}: \mu=100 \\H_{1}: \mu \neq 100\end{array}$

When testing the hypothesis presented below, at a .05 level of significance with the t test, where is the rejection region? $\begin{array}{l}H_{0}: \mu=100 \\H_{1}: \mu \neq 100\end{array}$

A) the upper tail
B) the lower tail
C) both the upper and tails
D) cannot be determined