Three cars move along a straight highway as follows: in one lane two police cars travel with 45 mph so that they are 300 feet apart with their sirens emitting simultaneoust sound at 890Hz (vmm : 340 mfs)' In the other lane a truck travels in the same direction with a speed of 35 mph. What beat frequency is observed by the truck passengers while the truck is passed by the first police car but not the second one (see figure}. Selectone: O at 7H2 O b. 16H2 O c. ZOHZ 0 d. 23 HZ