Why don't na+ ions move across the plasma membrane of neurons and down their concentration gradient all the time? a. the plasma membrane is not permeable to na+ ions. b. the na+/k+ pump is required and it only operates when atp is plentiful c. a na+ channel is required and it is usually closed. a & c. a, b, & c.

•there must be variation for the particular trait within a population.

•the variation must be inheritable (that is, it must be capable of being passed on from the parents to their offspring).

•individuals with one version of the trait must produce more offspring than those with a different version of the trait.good

explanation:

looks like the answer is     c     to me!