Consider a firms per-period (e.g., hourly) production process. If it employs 1 unit of labor, then 4 units of output will be produced; if it employs 2 units of labor, then 10 units of output will be produced; and if it employs 3 units of labor, then 18 units of output will be produced. It follows that: Total output is ______ at a(n) __________ rate and the marginal product of labor is ________.