Ocean swells generated in deep water have frequency \(f\) and travel at speed \(v_1\). As the waves approach a harbor entrance bounded by two breakwater walls separated by an opening of width \(D\), they enter a shallow region where their speed decreases to \(v_2\) while their frequency remains unchanged. Assuming the small-angle approximation \(\sin\theta \approx \theta\), which of the following expressions correctly represents the angle \(\theta\) from the central maximum to the first diffraction minimum of the waves entering the harbor?