Why, According to O'Neill, Does Utilitarianism Often Lack Precision

Why, according to O'Neill, does utilitarianism often lack precision?

A) Because it is concerned solely with justice, and it is never entirely clear what is just
B) Because it uses the obscure notion of happiness to think about moral problems
C) Because there is a limited amount of information available about the consequences of our actions
D) Because it relies on the notion of intention, and we can never fully know what people intend