18. Polyandrous mating systems are typically characterized by: high variance in female RS (reproductive success) relative to male variance in RS. high variance in male R$ relative to female variance in RS sexual monomorphism. Both a) and c) are correct. Borh b) and c) are correct. 19. Caffeine increases a person's alertness by: accelerating the enzymatic breakdown of adenosine in the synaptic cleft. synthesizing adenosine triphosphate from adenosine buildup within the synaptic cleft. preventing the synthesis of adenosine triphosphate in neuronal mitochondria. increasing the rate of adenosine re-uptake by terminal buttons. enabling action potentials that would otherwise be inhibited by adenosine. 20. Multiple sclerosis (MS) is a disease that affects the nervous system. In MS: myelin is gradually lost, leaving scar tissue and reducing neurons' ability to conduct electrical impulses and thereby send signals to muscles. neurotransmitter vesicles containing acetylcholine lose their ability to fuse with the pre-synaptic membrane, thereby reducing neurons ability to stimulate muscle contractions ion channels within axon membranes become blocked, preventing action potentials from being propagated. Nodes of Ranvier become larger, requiring neurons to expend signiticantly more energy to maintain the ion concentration differentials necessary tor generating action potentials. e) All of the above are TRUE