The correct answer is: A. Equal to 1.
The Cobb-Douglas production function is a mathematical function that describes the relationship between the inputs (capital and labor) and the output of a production process. The elasticity of substitution between inputs is a measure of how easily one input can be substituted for another. A Cobb-Douglas production function has an elasticity of substitution equal to 1, which means that the inputs are perfectly substitutable. This means that if the price of one input increases, the firm will be able to easily substitute another input without affecting the output.
Option B is incorrect because the elasticity of substitution between inputs is not always greater than 1. In fact, the elasticity of substitution can be less than 1, which means that the inputs are not perfectly substitutable. This means that if the price of one input increases, the firm will not be able to easily substitute another input without affecting the output.
Option C is incorrect because the elasticity of substitution between inputs is not always less than 1. In fact, the elasticity of substitution can be equal to 1, which means that the inputs are perfectly substitutable. This means that if the price of one input increases, the firm will be able to easily substitute another input without affecting the output.
Option D is incorrect because the elasticity of substitution between inputs is not always equal to 0. In fact, the elasticity of substitution can be greater than 0, which means that the inputs are not perfectly complementary. This means that if the price of one input increases, the firm will not be able to easily substitute another input without affecting the output.