A string of length `L` and force constant `k` is stretched to obtain extension `l`. It is further stretched to obtain extension `l_1`,. The work done in second stretching is
Options:
(a) `1/2kl_1(2l+l_1)`
(b) `1/2kl_1^2`
(c) `1/2k(l^2+l_1^2)`
(d) `1/2k(l_1^2-l^2)`