GATE 2025 ST – Question 12
Which statement about countability is correct?
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Correct answer: (B) The set {Σₙ₌₁∞a_n/10ⁿ:a_n∈{1,2}}⊂(0,1) is uncountable.
Explanation
A is a countable union of finite sets. B is in bijection with all infinite binary sequences, an uncountable set; its decimal representations using only 1,2 are unique. Each disjoint nonempty open interval contains a different rational, so C is countable. Rational endpoint pairs form a subset of ℚ², refuting D. **B**.