question: Find the spring constant of the bimetallic bar shown in <image 1> in axial motion.
  Options: A: $k_{eq}=4.80\cdot10^{7}\frac{N}{m}$, B: $k_{eq}=5.80\cdot10^{7}\frac{N}{m}$, C: $k_{eq}=6.80\cdot10^{7}\frac{N}{m}$, D: nan