question: The piece of rubber is originally rectangular and subjected to the deformation shown by the dashed lines. Determine the average normal strain along the side AD. <image 1>
  Options: A: 0.00680 mm/mm, B: -0.00680 mm/mm, C: -0.0281*10^(-3) mm/mm, D: 0.0281*10^(-3) mm/mm