[Solved] A variable-interval schedule is a schedule: A)by which reinforcement is given only after a specific number of responses are made. B)by which reinforcement occurs after a varying number of responses rather than after a fixed number. C)that provides reinforcement for a response only if a fixed time period has elapsed,making overall rates of response relatively low. D)by which the time between reinforcements fluctuates around some average rather than being fixed. Cycads: Ancient seed plants with a large crown of compound leaves and a stout trunk, often mistaken for palms or tree ferns. Ginkgo: A member of an ancient gymnosperm group that consists of a single living representative (Ginkgo biloba), a hardy, deciduous tree with broad, fan-shaped leaves and naked, fleshy seeds (on female trees) Nonmotile Sperm: Sperm cells that lack the ability to move or swim, which can affect fertilization capabilities. Pollen Grains: Microscopic structures that carry the male gametes of seed plants and are necessary for fertilization.