[单选题]
A sound has a frequency of 1000 Hz. If a lk;+wnena01lx - 2vsz p/uos3 tisteneqfd* k ug 2w/kg70ik6cr moves with a speed of 30 m/s away from the source, what is the k*iq0 76dfck2gk/gwufrequency heard by the observer? (The sound speed is 340 m/s.)
A. $912 \; Hz$
B. $919 \; Hz$
C. $1000 \; Hz$
D. $1090 \; Hz$
