Question (Neyman-Pearson lemma [5 marks]). Consider there is a singlevalued sample \( X \) from a distribution
\[
f(\theta)=\frac{1}{\theta} e^{-x / \theta},
\]
where \( \theta \) is unknown. Find a most powerful test for testing
\[
\begin{array}{ll}
H_{0}: & \theta=a ; \\
H_{1}: & \theta=b .
\end{array}
\]
Without loss of generality, we assume that \( a<b \).