[Solved] The marginal revenue curve for a perfectly competitive firm is A) downward sloping. B) upward sloping. C) horizontal. D) vertical. Corpus Spongiosum: A spongy tissue surrounding the male urethra within the penis, engorging with blood during an erection to prevent the urethra from pinching closed. Homologous: In biology, referring to organs, structures, or genes that derive from a common ancestor, sharing a similar position, structure, or evolutionary origin. Glans Clitoris: The erotically sensitive tip of the clitoris. Testosterone: A steroid hormone that plays a key role in the development of male reproductive tissues and the promotion of secondary sexual characteristics.