Natural selection is species survival and reproduction due to differences in some traits that give them an advantage over others
Polar molecules occur when two atoms do not share electrons equally in a covalent bond. While polar molecules have two parts with opposite charges (i.e., positive and negative), nonpolar molecules have no dipole, meaning that there is molecular symmetry, which results in no opposing charges.
Prokaryotes don't have a nucleus so it's bacteria :)
Cerebellum smoothes and coordinates the movement of skeletal muscle.
The second line of defense is nonspecific resistance that destroys invaders in a generalized way without targeting specific individuals: Phagocytic cells ingest and destroy all microbes that pass into body tissues. For example macrophages are cells derived from monocytes.
