Which term refers to the connective tissue membranes surrounding the brain and spinal cord?

Which beat describes the brain stem

Agene pool: a) is the collection of all the alleles found in a single species b) is the collection of all the alleles found in a single population c) is the collection of all the chromosomes found in a single population d) is the collection of genes in a species that allow the individuals to swim

When listening the levels of orginization in orginisims from the smallest to most complex , which level is just below organs in complexity