[单选题]
The wavelengths of the sounds pr 5 3jti q)d+ k1p4mpebjy7ew8ta;3*cnz 7hr a0 k9lsguoduced by two horns are 6 m and 7 m respectively. What beat frequency is heard when the horns are sounded on a day when the velocity of sound is 340 m/s?*9ugl3rak; hn c az0s7
A. $5 \; Hz$
B. $6 \; Hz$
C. $7 \; Hz$
D. $8 \; Hz$