Find the closure of the set instructor grade in school

Question

(Intersection) Express R ∩ S in terms of differences. 7.

(Closure) Let R be a relation and let X = {A₁, A₂, ... , A_k } be a subset of the attributes of R. Define X+, the closure of X, as the smallest set X+ such that X+ ⊃ X and X⁺ → B implies B ∈ X⁺.

(a) Find the closure of the set {Instructor, Grade} in School.

(b) Show that X is a super key if and only if X+ is the set of all attributes of R. Hint! For "if " construct X+ step by step.