Consider a random sample $X_1, \dots, X_n$ from a distribution with density $f(x|\theta) = \theta x^{\theta-1}$ for $x \in (0, 1)$. Suppose we want to test $H_0: \theta = 1$ versus $H_a: \theta = 2$. According to the Neyman-Pearson Lemma, which statistic provides the most powerful test?