A source emitting sound waves of constant frequency moves at a constant velocity along a horizontal line. The resulting circular wavefronts emitted by the source are shown in the diagram, along with two stationary observers at positions X and Y. Which of the following correctly identifies the direction of motion of the sound source and compares the sound frequency \(f_{\text{X}}\rangle detected at position X to the frequency \(f_0\) emitted by the source?