bone cells are arranged in concentric circles around longitudinal (lengthwise) tubes containing blood vessels. these longitudinal tubes in the center of each osteon are called?

An atom of nitrogen had seven protons in its nucleus. how many electrons does it have

Average annual precipitation and are the two major features that define the biomes on earth.

Some birds look for food near cattle and other livestock. the movement of the cattle churns up the ground, making insects available to the birds. the cattle appear to be unaffected by the presence of the birds. what kind of relationship exists between the birds and the cattle? o a. parasitism o b. commensalism o c. predation o d. mutualism