
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
According to the ABO blood group system theory, every person's blood type is one of A, B, O, or AB. According to the genetics of blood type inheritance, a child can never have blood type O if at least one of their parents has blood type AB. If a person has blood type O, then the number of all possible combinations of their parents' blood types is ___.

Present the answer in LaTex format: \boxed{Your answer}
user