Given the following premises, which of the following can be taken as justified by the rule given in a SINGLE step?
∼U⊃(S⋅K)
R⊃(∼U⋅∼U)
S≡∼U
​a. R⊃(S⋅K), from 1, 2, hypothetical syllogism
b. R⊃U, from 2, Double Negation
c. (∼∪⋅S)⊃K, from 1, Exportation
d. R⊃∼U, from 2, tautology