Consider the following sequence that follows some arrangement : c_accaa_aa_bc_b The letters that appear in the gaps are
- (a) abba
- (b) cbbb ✓ UPSC's answer
- (c) bbbb
- (d) cccc
Why the answer is (b)
• Filling the gaps with c, b, b, b gives c c a c c a a b a a b b c b b.
• Split into groups of five letters: ccacc | aabaa | bbcbb.
• Each group follows the same design: a pair of one letter, a different letter in the middle, and the same pair again (cc a cc, aa b aa, bb c bb), with the middle letter of one group becoming the pair of the next.
• Only 'cbbb' produces this regular pattern, option (b).
Why the other options are wrong
- (a) abba
- 'abba' gives caacc aabaa bbcab; the first and last groups break the pattern.
- (c) bbbb
- 'bbbb' gives cbacc aabaa bbcbb; the first group breaks the pattern.
- (d) cccc
- 'cccc' gives ccacc aacaa cbccb; the second and third groups break the pattern.
Asked in the CSAT Paper II of the UPSC Civil Services Preliminary Examination 2019, held on 2 June 2019. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.