Triangles $PQR$ and $STU$ are similar. Which of the following statements must be true?

I. The perimeter of $△PQR$ is equal to the perimeter of $△STU$.
II. The measure of $\angle P$ is equal to the measure of $\angle S$.
III. The ratio of $PQ$ to $ST$ is equal to the ratio of $QR$ to $TU$.