The only example of a positive feedback loop is labor contractions, since in that case, the output increases the original stimulus. In all of the other answer choices, the body is trying to maintain homeostasis by reversing the effect of the original stimulus. 

All life forms must be able to uptake or produce energy to survive (metabolism), grow, and reproduce to propagate the species. While having a safe place to live is an ideal for most species, it is not a requirement of life.

These criteria can be tested by thinking of a single cell. A single cell requires energy, and can metabolize it via glycolysis (even in an anaerobic environment). A cell can grow, and does so during the G1 phase of the cell cycle. A cell is also capable of reproducing via mitosis. A single cells does not, however, require shelter; some cells live in highly extreme environments.

An object is considered as 'living' if it is able to grow through metabolism, adapt to the environment, and reproduce. All organisms are also composed of cells.

Ability to interact with other life forms is not a requirement for life.

A helix is formed when Hydrogen bonds occur between the amino group in one peptide bond and the carboxyl group of another in the same polypeptide chain.

The four basic biological macromolecules carry out virtually every metabolic process of living organisms. Keep in mind that these molecules work together to achieve common goals. For example, enzymes (proteins) are used to help break down glucose (carbohydrate) in glycolysis. One product of glycolysis is energy in the form of ATP. ATP can be used to polymerize nucleotides (nucleic acids) to copy DNA.

The functions of specific types of macromolecules are highly dependent on their structures. For example, firbous proteins are used to provide structural support, while globular proteins are better suited to catalyze reactions as enzymes. The variety of macromolecular structures is directly related to the multitude of functions these molecules can facilitate.

All of these answers are characteristics of life. All organisms are capable of reproduction, respond to their surrounding environmental stimuli, process chemical energy, and grow and develop. This is true of organisms are any level, from animals like humans, down to the simplest prokaryotic bacteria.

All amino acids contain carbon, hydrogen, oxygen, and nitrogen. These elements create a carboxylic acid group and an amine group, which can fuse to form a peptide bond. Peptide bonds hold amino acids together and generate the primary structure of the protein.

Cysteine, a specific amino acid, also contains sulfur. Thus, the correct answer is that carbon, oxygen, hydrogen, nitrogen, and sulfur can all be found in amino acids.

Phosphorus is never found in amino acids, but plays an important role in the structure of nucleic acids, such as DNA, and in the modification and activation of proteins.

The R group is a side chain connected to the central carbon atom in an amino acid. The central carbon atom of an amino acid can bind to four other groups. In an amino acid, the central carbon will always bind to a carboxyl group, and amine group, and a hydrogen atom. The fourth bond, however, will be different for each amino acid, linking the central carbon to the R group.

Waxes provide skin with a waterproof coating. Waxes are made out of fats.

Hemoglobin is a protein responsible for transporting oxygen. DNA polymerase repairs DNA molecules. Amylase is the protein that helps convert starch to glucose.

Proteins are synthesized through dehydration synthesis reactions, which is the removal of water between two amino acids. In this case, two hydrogen atoms are removed from the amine group and one oxygen is removed from the carboxyl group, forming a peptide bond between the carbon atom of one amino acid and the nitrogen atom of the other amino acid.