When fatty acid molecules are in water, they can self-organize into a bilayer (double layer) that surrounds an inner core of water. Where "middle" refers to the middle of the bilayer between the outer and inner layers of fatty acids, which of the following statements is correct? A. The polar heads of the outer layer face the middle; the polar heads of the inner layer face toward the middle.
B. The polar heads of the outer layer face outward; the polar heads of the inner layer face toward the middle.
C. The polar heads of the outer layer face outward; the polar heads of the inner layer face toward the inner water core.
D. The polar tails of the outer layer face toward the middle; the polar tails of the inner layer face toward the inner water core.
E. The polar tails of the outer layer face outward; the polar tails of the inner layer face toward the middle.

C. The polar heads of the outer layer face outward; the polar heads of the inner layer face toward the inner water core.

Explanation:

Lipids characterize for being amphipathic molecules, which means that they have a hydrophilic portion and a hydrophobic portion at the same time. These molecules have a lipidic head that corresponds to a negatively charged phosphate group, which is the polar and hydrophilic portion. They also have two lipidic tails that correspond to the hydrocarbon chains -the apolar and hydrophobic portion- of the fatty acids that esterify glycerol.  When fatty acid molecules are in water they are arranged with their hydrophilic polar heads facing the exterior and the interior of the core of water, while their hydrophobic tails are against each other, constituting the internal part (middle).  

any abiotic or biotic factor in an ecosystem that causes a population's size to slow or decrease is a limiting factor.