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 in the spellings of entire English Counting
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