If 10 J of Work Are Needed to Stretch a Spring

If 10 J of work are needed to stretch a spring from 10 cm to 12 cm and another 20 J are needed to stretch it from 12 cm to 14 cm, what is the natural length of the spring? Round the answer to nearest integer.

A) $10 \text { cm }$
B) $11 \text { cm }$
C) $6 \mathrm{~cm}$
D) $8 \mathrm{~cm}$
E) $9 \mathrm{~cm}$