Which pair of vectors is orthogonal?

⟨3, –5⟩ and ⟨6, –10⟩
⟨2, –3⟩ and ⟨9, –6⟩
⟨–2, 3⟩ and ⟨–6, –4⟩
⟨–5, 4⟩ and ⟨–4, –2⟩