A source of sound of frequency\[{{f}_{1}}\] is placed on the ground. A detector placed at a height is released from rest on this source. The observed frequency\[{{f}_{{}}}(Hz)\] is plotted against time \[t\](sec). The speed of sound in air is 300 m/s. Find\[{{f}_{1}}\]\[(g=10m/{{s}^{2}})\]

• A. \[0.5\times {{10}^{3}}Hz\]       
• B. \[1\times {{10}^{3}}Hz\]
• C. \[0.25\times {{10}^{3}}\]         
• D. \[0.25\times {{10}^{3}}Hz\]