The function \(f\) is continuous for all real numbers, and the average rate of change of \(f\) on the closed interval \([2,5]\) is \(-\dfrac{5}{2}\). For \(2\lt c\lt 5\), there is no value of \(c\) such that \(f'(c)=-\dfrac{5}{2}\). Of the following, which must be true?