Assertion (A): Any factor that affects a firm's marginal cost curve is of course a determinant of its supply curve. This is true. A firm's supply curve is directly related to its marginal cost curve. Changes in factors that affect marginal cost (e.g., input prices, technology) will also affect the firm's supply decisions.

Reasoning (R): A firm's supply curve is a part of its marginal cost curve. This is also true. Specifically, in a perfectly competitive market, the firm's supply curve is the portion of its marginal cost (MC) curve that lies above its average variable cost (AVC) curve.

Relationship: The reasoning correctly explains the assertion. Since the supply curve is derived from the marginal cost curve, any factor affecting the MC curve will inherently affect the supply curve.