10) Which of the following inference system is invalid?
a) R---> S, ~S |- ~R
b) ~R, P----> Q, Q -----> R |- ~P
c) ~R (S --------> ~T), ~R ∨ W, ~P --------> S, ~W |- T ------> P
d) P ∧ Q -----> ~T, W ∨ R, W -----> P, R -----> Q |- (W ∨ R) --------> ~T