[填空题]
A factory whistle emits fg1tc:b0 zk: -.i 8z frhmbk1m1f r)qcsound olcj:4lx5(*4avlq q gnf frequency 570 Hz. When the wind velocity is 12 m/s from the45lclgn*4 axjql v(: q north, what frequency will observers hear who are located, at rest,
(a) due north, Hz
(b) due south, Hz
(c) due east, Hz
(d) due west, of the whistle? What frequency is heard by a cyclist heading Hz
(e) north Hz
(f) west, toward the whistle at 15 m/s? Hz
Assume T = 20 $^{\circ} $C