When we are born, we have about how many brain cells? none, they develop after birth hundreds millions billions

Millions

Explanation:

Neurons are small brain cells that perform functions related to the nervous system. In our brain there are millions of neurons, scientists estimate we have about 80 million when we were born. As we grow, this number decreases. After age 80, we will have lost 30% of our neurons. Throughout life, we lose and regenerate neurons constantly. Through the process of regeneration of the neruons, new connections are generated that produce a process called neurogenesis. This process allows the creation of new neurons during one's life.

what effect does exercise have on the nervous system?

the answer is c.