All you have to remember is that when the source is APPROACHING the observer,
the frequency seems HIGHER than what's actually leaving the source.
The siren is wailing at 1,000 Hz.
(1). 30.0 Hz . . . that's way lower
(2). 9.19 x 10² Hz = 919 Hz . . . that's lower
(3). 1.00 x 10³ Hz = 1,000 Hz . . . that's exactly the same
(4). 1.10 x 10³ Hz = 1,100 Hz . . . that's the only choice that's higher than 1,000 Hz.
