An element has two isotopes, A and B, with mass numbers of 32.0 and 33.0 respectively. The relative atomic mass is 32.3. The percentage abundance of the two isotopes is:

a) A - 20% B - 80%

b) A - 25% B - 75%

c) A - 80% B - 20%

d) A - 70% B - 30%

e) A - 50% B - 50%