[Solved] The marginal cost of investment for the firm is equal to A) 1. B) 0. C) the depreciation rate. D) the depreciation rate plus the interest rate. Self-Confident: A feeling of trust in one's abilities, qualities, and judgement. Empathic: The ability to understand and share the feelings of another, essentially putting oneself in another's shoes emotionally. Middle Adulthood: A stage of life typically ranging from about 40 to 65 years of age, characterized by increased focus on personal and professional achievement, stability, and health. Erotic Plasticity: The degree to which an individual's sex drive can be changed by social, cultural, or situational factors.