Economists who study production of goods by a firm consider two functions. The revenue function $R(x)$ is the revenue the firm receives when x number of units are sold. The cost function $C(x)$ is the cost the firm incurs when producing x number of units. The derivatives of these functions $R'(x)$ and $C'(x)$ are called by economists the marginal revenue and cost function. What does $\int_{1000}^{\,5000} {R'(x)}\,dx$ represent?

A. The increase in revenue when production is increased from 0 units to 5000 units.
B. The increase in revenue when production is increased by 1000 units.
C. The increase in revenue when production is increased by 5000 units.
D. The increase in revenue when production is increased from 1000 units to 5000 units.