A, B, C, and D are all very uncommon in English spellings of numbers

The letter ‘D’ does not appear until the number 100 (hundred). The letter ‘A’ is not used until the number 1,000 (thousand). ‘B’ does not show up until 1,000,000,000 (billion), and ‘C’ is not found until 1 octillion! That's 10 to the 27th power or 1,000,000,000,000,000,000,000,000,000.