Final answer:
Option D, 'It increases the amount productive resources in the economy,' is not a reason why there are gains to specialization.
Step-by-step explanation:
In economics, the gains to specialization are numerous. However, one of the options provided is not a reason why there are gains to specialization. Option D, 'It increases the amount productive resources in the economy,' is not a reason why there are gains to specialization. Specialization focuses on allowing individuals to concentrate on the activities in which they have a comparative advantage, improving skills through experience and practice, and eliminating the costs of switching from one task to another. Increasing the amount of productive resources in the economy is not directly related to specialization.