[单选题]
The wavelengths of the )k en, sg a ;ildro4*5ym(vq*usounds produci.l dce g6:zq 6y5v5keed 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 souncyqeeg5 vkdi5:z 6l6 .d is 340 m/s?
A. $5 \; Hz$
B. $6 \; Hz$
C. $7 \; Hz$
D. $8 \; Hz$
