20.K is the force constant of a spring. The work done in increasing its extension from

20.K is the force constant of a spring. The work done in increasing its extension from l₁ to l₂ will be

(a)   k(l₂ – l1 )                                                  (b) (k/2)(l₂ + l1 )

(c)    k( l₂ ² – l1 ² )                                           (d)  (k/2)( l₂ ² – l1 ² )

Answer-d