The length of an elastic string is a metre when the longitudinal tension is $4\, N$ and $b$ metre when the longitudinal tension is $5\, N$. The length of the string in metre when the longitudinal tension is $9\, N$ is
  • A$a - b$
  • B$5b - 4a$
  • C$2b - \frac{1}{4}a$
  • D$4a - 3b$
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