Which of the following valves has two leaflets and is closed during the systolic phase of the cardiac cycle?

a. Pathogen: A pathogen is a disease-causing organism like bacteria, virus, or parasite. It can trigger an immune response, but not necessarily a hypersensitivity reaction.

b. Allergen: An allergen is a foreign substance (like pollen, dust mites, or food) that can trigger an immune response in an already sensitized individual. This response can lead to allergy symptoms like hay fever or anaphylaxis

c. Immunogen: An immunogen is any substance that can trigger an immune response, but not necessarily a hypersensitivity reaction. It can be a pathogen, allergen, or other foreign substance.

d. Fibrinogen: Fibrinogen is a blood protein involved in blood clotting, not allergic reactions.

a. Cartilage: Cartilage is a type of connective tissue found at the ends of bones in joints. It provides a smooth, elastic surface that cushions the bones and reduces friction during movement.

b. Tendon: Tendons are tough, fibrous bands that connect muscles to bones. They do not reduce friction at joints.

c. Fibrillin: Fibrillin is a protein component of connective tissue, but it's not a specific structure like cartilage and doesn't directly reduce friction in joints.

d. Osteocytes: Osteocytes are mature bone cells. They are not located at joints and don't play a role in friction reduction.