Two high-speed military jets, Jet 1 and Jet 2, fly along parallel horizontal flight paths separated by a perpendicular distance \(d\). Jet 1 moves to the right at a constant speed \(v_j\) and continuously emits a sound signal of frequency \(f_0\). Jet 2 moves to the left at the same constant speed \(v_j\) and is equipped with a sound detector. The speed of sound in air is \(v\), where \(v > v_j\). At the instant shown, the line of sight connecting Jet 1 to Jet 2 makes an angle \(\theta\) with Jet 1's direction of motion (where \(0 < \theta < 90^\circ\)). Which of the following expressions correctly represents the signal frequency \(f'\) detected by Jet 2 at this instant?