Gamma gamma gameleon

Calculus

Which of the following is/are used in the derivation of

$(s-1)! \equiv \int_{0}^{\infty} e^{-t}t^{s-1} \, dt\, ?$

A. $\$ Integration by parts
B. $\$ U-substitution
C. $\$ Induction

A B C A and B A and C B and C All of them None of them