Given a scientific question and its correct answer, generate supporting facts for the answer. This supporting fact can be an explanation for the answer to the given question.

[Q]: Question: What remains a constant of radioactive substance over time? Answer: rate of decay
[A]: The rate of decay of a radioactive substance is constant over time.

[Q]: Question: Long distance runners try to maintain constant velocity with very little acceleration or deceleration to conserve what? Answer: energy
[A]: Long distance runners try to maintain constant velocity with very little acceleration or deceleration to conserve energy.

[Q]: Question: Desert plants have evolved processes to conserve water and deal with harsh conditions. a more efficient use of co2 allows plants to adapt to living with what? Answer: less water
[A]:
perpendicular to the surface at the point where the ray strikes the surface.