What is relative fitness? the ability to survive and reproduce survival of the fittest the contribution an individual makes to the gene pool of the next generation relative to the contribution of other individuals differences among individual in the composition of their genes or other dna sequences differences in dna sequence that do not confer a selective advantage or disadvantage?

The answer would be:  the contribution an individual makes to the gene pool of the next generation relative to the contribution of another individual

Relative fitness is the capability for an organism to reproduce and crate viable offspring (offspring live to maturity and which can reproduce themselves). It is  the survival rate relative to the maximum survival rate with other population.

For example if organism A ends up finding a mate and have babies and organism B does not this means that organism A has more fitness biologically than organism B.  

b is the best answer for this question.

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: