I have to give examples of four kinds of subsequences if they exist. S∈ℝ is the limit point of a subsequence of the sequence (Xn) (n=1 -> ∞) if ∃ subsequence (Xnk) (k=1 -> ∞) : Xnk -> S.

What could the sequences be or do they exist if

a) limit point of subsequence = {0}

b) limit point of subsequence = {0,1}

c) limit point of subsequence = {infinitely many points}

d) limit point of subsequence = Q ∩ [0,1]