[单选题]
The wavelengths of the sounds prdm8iz4).c*g (cyumdn c6ms5ihrh-7 iv sm+c. oduced by two horns are 6 m and 7 m respectively. What b 76i-rhcc hm+5s.smvieat frequency is heard when the horns are sounded on a day when the velocity of sound is 340 m/s?
A. $5 \; Hz$
B. $6 \; Hz$
C. $7 \; Hz$
D. $8 \; Hz$
