Answer: The number of students that there blood type is type O
Explanation:
With simple analogy, Abel is a member of a class, and Abel was not hurt , he is a victim.
The perpetrator is a member of a class (fact 1) and the blood type found on the crime scene is type O(fact 2). To draw conclusion,
The perpetrator is possibly someone from the class with a type O blood type. So a narrow research should be carried out on the blood type of the students in a class and to be more precise , someone who just got hurt , a clear injury and has a blood type of type O would be the possible perpetrator.