Question

Three isotopes of an element have mass numbers, M, (M + 1) and (M+2) . If the mean mass number is (M +0.5) then which of the following ratios may be accepted for M, (M + 1) , (M + 2) in that order?
A.1:1:1
B. 4:1:1
C.3:2:1
D.2:1:1