This is one of those moments when I'm glad I'm not a teacher anymore. If you try to be funny or clever (and you're not careful), then you'll write something like this on a practice test (quoted from First Coast News):
Ursula was angry after her boyfriend broke up with her after having
sex. To get revenge, she had sex with his best friend the next day.
Ursula had a beautiful baby girl nine months later. Ursula has type O
blood, her ex-boyfriend has type AB blood and his best friend is type A
blood. If her baby daddy is her ex-boyfriend what could the possible
blood type(s) of her baby NOT be.