‘a‘ does not appear anywhere in the spellings of 1 to 999, it comes for the first time in Thousand
‘b‘ does not appear anywhere in the spellings of 1 to 999,999,999, it comes for the first time in Billion
‘c‘ does not appear anywhere in the spellings of entire English Counting
‘d‘ does not appear anywhere in the spellings of 1 to 99, it comes for the first time in Hundred