Why are people with O blood group called as universal donors?

Why are people with AB blood group called as " Universal Recipients "?

Which blood group is called universal donor ?

Why is the blood group 'O' called as universal donor?

Why is a person having blood group AB called universal recipient and a person with blood group O called a universal donor?

Why is person with O-blood group called universal donor?

Which blood group is called Universal donor?

Give reason: People with O^- blood group are called universal donor.

Person with "O" blood group is called universal donor. Why?

_____blood group is called universal donor.