Why is it even more dangerous to the user when heroin is mixed with quinine?

A) Chemicals in quinine interact with heroin to make the effects stronger.
B) Too much quinine is toxic to the body.
C) Because heroin and quinine both are bitter-tasting, the user can't tell how potent the heroin is.
D) Some users are allergic to quinine and can die from an allergic reaction.

Answer: C