interesting observation

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