show that an open rectangle is an open set [on hold]

Show that an “open rectangleâ€‌ (a,b)×(c,d)={(x,y)∣x∈(a,b),y∈(c,d)}(a, b) \times (c, d) = \{(x, y)\mid x \in
(a, b), y \in (c, d)\} is an open set in R2\mathbb R^2.

This is my homework and I have no idea on how to start this question. Please help!

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