Multi-choice problem: Continue writing the next sentence in this paragraph:

How to test pool water
Run the pump in the morning for 15 minutes.
Aim on getting there before noon, since the day's heat and sunlight affect the chemicals in the water. If the pool's systems were turned off, turn them on and wait at least 15 minutes for the chemicals to disperse.

Choose your answer from:
[a]. If your home doesn't come with a pool turn-on or you risk becoming dependent on chemicals, you may want to give it more time. However, a pool at 40 feet (4.4 m) in length may require 15 minutes.
[b]. There is no need to test water for chlorine. However, that particular chemical can help your pool stay that way.
[c]. Doing so ensures that the water stays fresh and won't be contaminated by hydrogen peroxide. Clean your pool with a chlorinated or bleach solution.
[d]. As long as the systems were kept on for at least 15 minutes, you can collect the sample immediately. Take the sample in the middle of the pool.
[d].