If two parents who's blood types are both b produce a child that has blood type o, what can you say about the parent's genotypes?
a. one is ii
b. one is ibi
c. both are ibi
d. nothing, this is impossible

Our blood groups are composed of two "letters", one from both parets.

People with Blood group B can have the type BB or B0 (IBi).

if they both have a blood group B0 (that is IBi) they can both give their children the "0" part of their blood group, and the child will have a 0 blood group.

The answer is

c. both are IBi (that is both are B0).

Cherries for sure do doubt