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Which two expressions count the number of subsets of an $n$-element set?

Select ALL that apply.

$\sum_{k=0}^n{n\choose 2}$

$\sum_{k=0}^n{n\choose k}$

$\sum_{k=0}^n 2^k$

${n\choose 2}$

$2^n$

$n^2$