A hollowpipe of length 0.8 mis closed at one end. At its open end a 0.5 mlong uniform string is vibrating in its second harmonic and it resonates with the fundamental frequencyof the pipe. If the tension in the wire is 50 N and the speed of sound is 320 ms$$^{-1}$$, the massof thestring is

A

5 grams

B

10 grams

C

20 grams

D

40 grams