did you know?in the decimal number system(base 10

did you know?in the decimal number system(base 10
) ten different digits,0 to 9,are used to write all the numbers.in the binary number system(base 2) two different digits are used,i.o.and 1.
which one of the following numbers is not a valid number in the octal number system(base 8)?
a 128 b 127 c 126 d 125 e 124