In a guitar, two strings $A$ and $B$ made of same material are slightly out of tune and produce beats of frequency $6\text{Hz}$. When tension in $B$ is slightly decreased, the beat frequency increases to $7\text{Hz}$. If the frequency of $A$ is $530\text{Hz}$, the original frequency of $B$ will be:

1. A
   $523\text{Hz}$
2. B
   $524\text{Hz}$
3. C
   $536\text{Hz}$
4. D
   $537\text{Hz}$