The length of elastic string, obeying Hooke's law is \[{{\ell }_{1}}\]metres when the tension 4N and\[{{\ell }_{2}}\]metres when the tension is 5N. The length in metres when the tension is 9N is -

• A. \[5{{\ell }_{1}}-4{{\ell }_{2}}\]
• B. \[5{{\ell }_{2}}-4{{\ell }_{1}}\]
• C. \[9{{\ell }_{1}}-8{{\ell }_{2}}\]
• D. \[9{{\ell }_{2}}-8{{\ell }_{1}}\]