The pericardial cavity separates the parietal and visceral pericardium
The correct answer is proteins.
Proteins have four structures; (1) primary, (2) secondary, (3) tertiary, and (4) quaternary. Primary structures of proteins are formation of peptides and the peptide bonds between amino acids. Secondary structures of proteins involves the amino acid composition leading to hydrophobic interactions forming either alpha helices or beta sheets. Interactions between secondary structures of proteins will make the tertiary structure which is either soluble (globular) or insoluble (fibrous) proteins. Quaternary structures are the combination of two or more tertiary structures and these are called subunits.
The answer is A.
Twins form in one of two ways:
Identical twins occur when a single fertilized egg splits into two.
Fraternal twins occur when two, separate eggs are fertilized by two, separate sperm.
The cell membrane helps to control what enters and leaves a cell. The substance which accounts for two-thirds of a cells mass is water. That makes the answer D.