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ISRO-DEC2017-21

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For $\Sigma =\{a,b\}$ the regular expression $r=(aa)^{*}(bb)^{*}b$ denotes

  1. Set of strings with $2\,{a}'s$ and $2\,{b}'s$
  2. Set of strings with $2\,{a}'s$ $2\,{b}'s$ followed by $b$
  3. Set of strings with $2\,{a}'s$ followed by ${b}'s$ which is a multiple of $3$
  4. Set of strings with even number of ${a}'s$ followed by odd number of ${b}'s$
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Since Kleene star (*) is used,  the string b should be accepted by  r=(aa)∗(bb)∗b

Voila!!! Options A, B and C are eliminated in one shot!!

answer is D

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