Uncountable or countable

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For each $t\in R$, let $E_t$ be a subset for $R$. Suppose that is $s<r$ then $E_s$ is a proper subset of $E_r$ . Must be $\bigcup_{t\in R}{E_t}$ be uncountable?

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No. Consider the set of rationals less than a given real. That is, take $$E_r=\{q\in\Bbb{Q}:q<r\}.$$