The Mass in the Figure Below Slides on a Frictionless $-\frac{k_{1}+k_{2}}{2}\left(x_{1}+x_{2}\right)$

Multiple Choice

The mass in the figure below slides on a frictionless surface. When the mass is pulled out, spring 1 is stretched a distance x1 from its equilibrium position and spring 2 is stretched a distance x2. The spring constants are k1 and k2 respectively. The force pulling back on the mass is:

A) (-k2x1.)
B) (-k2x2.)
C) (-(k1x1 + k2x2) .)
D) $-\frac{k_{1}+k_{2}}{2}\left(x_{1}+x_{2}\right)$ .
E) $-\frac{k_{1}+k_{2}}{k_{1} k_{2}}\left(x_{1}+x_{2}\right)$ .