A train moving at $30$ m$/$s approaches a stationary observer. The train's whistle emits a frequency of $500$ Hz$.$ Calculate the observed frequency of the whistle. $($Speed of sound in air $=340$ m$/$s$).$

B$.$ If the train is moving away from the observer at the same speed, what frequency will the observer hear?

C$.$ Discuss how the Doppler Effect is applied in everyday technologies, such as radar or medical imaging.