Study the given pie-chart and answer the question that follows.
The pie-chart shows the data for A, B, O , AB blood groups of 240 donors.

The ratio of the donors of blood group 'O' to the average of the donors of groups 'A' and 'AB' is:

A

1 : 3

B

1 : 1

C

1 : 2

D

2 : 1