The marginal revenue product (MRP) of land declines as more land is brought into production because:

A) land is a "free and nonreproducible gift of nature."
B) of diminishing returns.
C) land rent has no incentive function.
D) the supply of land is fixed.

The principle of diminishing returns states that as more units of a variable input (such as land) are added to a production process with a fixed amount of other inputs (such as capital and labor), the marginal product of that input (in this case, the marginal revenue product of land) will eventually decrease. Therefore, as more land is brought into production, the MRP of land will decline due to diminishing returns. The other options (A, C, D) are not directly related to the concept of diminishing returns and do not explain why the MRP of land would decline.