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Letters 'a', 'b', 'c' &'d' do not appear anywhere in the spellings of 1 to 99.
( Letter 'd' comes for the first time in Hundred )

Letters 'a', 'b' & 'c' do not appear anywhere in the spellings of 1 to 999
( Letter 'a' comes for the first time in Thousand )

Letters 'b' & 'c' do not appear anywhere in the spellings of 1 to 999,999,999
( Letter 'b' comes for the first time in Billion )
And
Letter 'c' does not appear anywhere in the spellings of entire English counting
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