How does the biological species concept identify two different species?
a. reproductive isolation
b. asexual reproduction
c. interaction between two species

The biological species concept identify two different species by reproductive isolation.  

Explanation:

In biological Species theory if organisms can interbreed and produce healthy and fertile offspring they belong to same specie. Speciation is the process of emergence of new specie from the existing population. When a group of organisms within a specie become reproductively isolated it give rise to the new specie.

answer: the blood-brain barrier is a layer of specialized membranes and cell processes designed to keep undesirable substances from passing from the bloodstream into the precious tissues of the brain. while the blood-brain barrier does keep most bad things out of the central nervous system, it is not impermeable.

explanation:

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."