Have you ever noticed ?

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