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