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

a. Insulin and glucagon: These hormones have opposite effects on blood glucose levels; insulin decreases blood glucose by facilitating cellular uptake, while glucagon increases blood glucose by stimulating glycogen breakdown and gluconeogenesis.

b. Ghrelin and gastrin: Ghrelin stimulates appetite, and gastrin stimulates gastric acid secretion, but they do not directly oppose each other.

c. Calcitonin and melatonin: Calcitonin lowers blood calcium levels, while melatonin regulates sleep-wake cycles; they do not act as antagonists.

d. Androsterone and estradiol: These are sex hormones with different roles but are not direct antagonists; they do not have opposing effects in the body.

a. Elastic connective tissue: This tissue contains elastic fibers and fibroblasts, providing elasticity and flexibility but not the tightly packed, parallel collagen fiber arrangement.

b. Dense irregular connective tissue: This tissue has collagen fibers arranged in a random, irregular pattern, providing strength in multiple directions but not the parallel arrangement.

c. Reticular connective tissue: This tissue contains reticular fibers forming a network to support soft organs but does not have densely packed collagen fibers.

d. Dense regular connective tissue: This tissue has fibroblasts packed tightly between parallel arrays of collagen fibers, providing great tensile strength along the direction of the fibers. It is found in tendons and ligaments.