(a) The alkene has a double bond between the second and third carbon atoms. In addition polymerization, the double bond breaks and forms a single bond with another monomer. Therefore, the repeat unit is CH₂-CH₂.

(b) The repeat unit is CH₂-CH₂. This means that the polymer chain is formed by repeating this unit. The structure of the addition polymer is:

-CH2-CH2-
|       |
-CH2-CH2-
|       |
-CH2-CH2-

The statement "The tertiary structure of a protein is primarily determined by the hydrophobic effect" is true, although it's not the *only* factor. The hydrophobic effect is a major driving force in protein folding and plays a crucial role in determining the tertiary structure.

Hydrophobic Effect: Amino acids with non-polar (hydrophobic) side chains tend to cluster together in the interior of the protein, away from the surrounding water. Water molecules are polar and form hydrogen bonds with each other. When non-polar molecules are exposed to water, the water molecules around them become more ordered and less able to form hydrogen bonds. This leads to a decrease in entropy (disorder) of the water. To minimize this disruption of the water's hydrogen bonding network, the non-polar molecules aggregate in the interior of the protein, minimizing their contact with water. This aggregation is energetically favorable and drives the folding process.

Role in Protein Folding: The hydrophobic effect is a key driving force in the folding of polypeptide chains into their unique 3D structures. As the polypeptide chain folds, hydrophobic amino acid side chains bury themselves in the protein's interior, minimizing their exposure to water. This process is often referred to as "hydrophobic collapse." The hydrophobic effect is often coupled with other interactions, such as hydrogen bonds and ionic bonds, to stabilize the folded protein. While other forces are important, the hydrophobic effect is often the primary driving force that dictates the overall shape and stability of the tertiary structure. Without the hydrophobic effect, proteins would not be able to fold correctly and would likely remain unfolded and inactive.

Burning plastic waste results in the formation of a variety of toxic gases, posing significant health hazards. The combustion process does not always completely burn the plastic, leading to the release of harmful substances. Carbon monoxide (CO) is a highly toxic, odorless gas that reduces the oxygen-carrying capacity of the blood, leading to asphyxiation. Dioxins and furans are highly persistent organic pollutants formed during incomplete combustion, particularly in plastics containing chlorine. These are known carcinogens and can cause a range of health problems, including immune system suppression, reproductive issues, and developmental problems. Hydrogen chloride (HCl) is a corrosive gas that can irritate the respiratory system, eyes, and skin. Exposure can cause coughing, choking, and severe burns. Particulate matter (PM2.5), tiny solid or liquid particles suspended in the air, is also released. These can penetrate deep into the lungs and bloodstream, contributing to respiratory illnesses, heart disease, and even cancer. Exposure to these toxic gases can have both short-term and long-term health effects, impacting respiratory, cardiovascular, and neurological systems. The health hazards are particularly severe for vulnerable populations such as children, the elderly, and individuals with pre-existing respiratory conditions.