If a function y = f(x) is differentiable, its derivative f '(x) is a function of x. If the derivative f '(x) is differentiable, the derivative of f '(x) is called the second derivative of y = f(x) and is denoted by y¨=f ¨(x). The derivative of the second derivative, i.e., f ¨(x) is called the third derivative which is denoted by y'''= f ''' (x). For a constant function f(x) = c the derivative is zero, that is, (c)' = 0. The slope of the tangent line to a curve y = f(x) at (x0, f (x0 )) is m = f ' (x0). For the identity function f(x) = x, the derivative is 1, that is, (x)' = 1. The answer is A.