Question

opiates, such as heroin, chemically resemble ______ and act as a powerfully pain killer.
seritonin
endorphines
dopamine

Explanation:

Opiates like heroin chemically resemble endorphins (natural pain - relieving and mood - regulating substances in the body) and can act as powerful painkillers by interacting with the same receptors that endorphins do. Serotonin is involved in mood, sleep, etc., and dopamine is related to reward and motivation, so they don't match the chemical resemblance and pain - killing mechanism of opiates.

Answer:

B. Endorphines