Answer: both labor input and capital input are increased proportionally.

**Explanation:** In the Cobb–Douglas production function, diminishing marginal productivity occurs when one input is increased while holding other inputs constant. However, when both labor and capital are increased proportionally (i.e., maintaining the same capital-to-labor ratio), the function exhibits constant returns to scale, and diminishing marginal productivity does not occur. This is because proportional increases in inputs lead to proportional increases in output, maintaining constant marginal productivity.

Author: LeetQuiz Editorial Team