What structure is part of an echinoderms water vascular system?

Why do we perceive objects as retaining their brightness under different lighting levels?

According the inverse square law, doubling the distance from the source of the sound, a speaker, for example, will drop the sound 6 db each time. if you were standing in the back of an auditorium, 32 feet away from a speaker not using any amplification, would you be able to hear a speaker clearly? why or why not?

Describe the structure of amino acid, including what makes amino acids different from eachother