How often should engage in an aerobic activity to reach an adequate level of cardiovascular fitness?

The concept of keystone species is controversial among ecologists because most organisms are highly interdependent. if each of the trophic levels is dependant on all others how can we say one is most important

Cells must be in osmotic equilibrium with their surroundings environments because if they swell or shrink thier membranes will rupture

Which u.s. state lists the horned toad as its state reptile?