What are two properties of a good marker and why are good markers important?

One group of reptiles, characterized by the fossil ark metal x

Which element forms the basis of all organic organisms a carbon b oxygen c hydrogen d nitrogen

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