The length of an elastic string is $'a'$ metres when the longitudinal tension is 4 N and $'b'$ meters when the longitudinal tension is 5 N. The length of the string in metres when the longitudinal tension is 9 N is :

Question

( A ) $4a - 3b$
( B ) $2b - \frac{1}{4}a$
( C ) $a-b$
( D ) $5b - 4a$