( 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