A hollow pipe of length $0.8 \mathrm{~m}$ is closed at one end. At its open end a $0.5 \mathrm{~m}$ long uniform string is vibrating in its second harmonic and it resonates with the fundamentals frequency of the pipe if the tension in the wire is $50 \mathrm{~N}$ and the speed of sound is $320 \mathrm{~ms}^{-1}$, what is the mass of the string.
(A) $20 \mathrm{~g}$
(B) $5 \mathrm{~g}$
(C) $40 \mathrm{~g}$
(D) $10 \mathrm{~g}$