please how do i determine if this is a subspace or not

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1. Is $\vec{0} \in U$?

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2. Suppose two vectors $\vec{a}, \vec{b} \in U.\$
   Is $\vec{a} + \vec{b} \in U$?

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3. Suppose some scalar $\lambda \in \mathbb{R}$ and some vector $\vec{a} \in U.\$
   Is $\lambda \cdot \vec{a} \in U$?