What are the major categories of health risks?

Johnny rode his bike to a friend's house 4 blocks down the street in his neighborhood. he immediately rode back home once he realized his friend was not able to play. what was his displacement for the total bike ride? how did you determine this? what could we use as a reference point to determine he was in motion during his bike ride? why can you use it as a reference point

Explain what a zygote is. use the terms egg cell, sperm cell, and fertilization in your explanation.

What is the term for a water wave that is created by an underwater earthquake?