The Concepts of Stock and Flow Are Involved in Firms

The concepts of stock and flow are involved in firms' demand for labour and for capital. Which of the following statements is correct?

A) A firm hires a flow of labour services; and purchases a stock of physical capital, which provides a flow of services.
B) Labour and capital are both flow variables because their values change over time.
C) Labour and capital are both stock variables because they are providing service at any given moment in time.
D) A firm's demand for both labour and capital is determined by the marginal value each provides to the stock of labour and capital, respectively.
E) A firm hires a stock of labour services; and purchases a stock of physical capital, which provides a flow of services.