The correct answer is: A. Asymptotics are used for inference usually.
Asymptotics are used to study the behavior of a function as its input approaches a certain value. They are not used for inference, which is the process of drawing conclusions about a population based on a sample.
B. Adding squared terms makes it continuously differentiable at the knot points. This is true. Adding squared terms to a polynomial makes it continuously differentiable at the knot points.
C. Adding squared terms makes it twice continuously differentiable at the knot points. This is also true. Adding squared terms to a polynomial makes it twice continuously differentiable at the knot points.
D. None of the mentioned. This is not true. Option A is the wrong statement.