A smaller person would have a lower metabolism rate than a taller person.
I think the hydra lacks the specialized cells for touch and chemical detection. On the other hand earthworms have touch, light, vibration and chemical receptors along the entire body surface. Both of them are invertebrates (those animals without a backbone), they include insects, worms, jelly fish, spiders among other animals.
Stratum corneum is the answer
A. Decreasing amount of energy needed to start a reaction.