Which of the following is a subgroup of the group $G = \{2^n \mid n \in \mathbb{Z}\}$ under multiplication?

Question

$\{4^n \mid n \in \mathbb{N}\}$

$\{3^n \mid n \in \mathbb{Z}\}$

$\{6^n \mid n \in \mathbb{N}\}$

$\{4^n \mid n \in \mathbb{Z}\}$

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