For a differentiable function f, a student uses the tangent line at x=1 to approximate f(1.1). The tangent-line calculation gives 2.80, but the actual value, found with technology, is f(1.1)=2.78. Which of the following best describes the concavity of f near x=1?
a) f is concave up because the approximation is an overestimate.
b) f is neither concave up nor concave down near x=1 since the approximation error is small.
c) f is concave down because the approximation is an overestimate.
d) f is concave down because the approximation is an underestimate.