A spring of negligible mass and length l₀ hangs from a fixed point. When a mass m is attached to the free end of the spring, the length of the spring increases to l. The tension in the spring is equal to k∆x, where k is a constant and ∆x is the extension of the spring. What is k?

 

A. $\frac{{mg}}{{{l_0}}}$

 

B. $\frac{{mg}}{l}$

 

C. $\frac{{mg}}{{l - {l_0}}}$

 

D. $\frac{{mg}}{{{l_0} - l}}$