Which substance is not normally used as a diluent of heroin?

If the singular term ends in -us, the plural is usually formed by changing the ending to

Every person’s body is different, a. but every vehicle’s cabin layout is the same b. and every vehicle’s cabin layout is different c. making some people automatically safe drivers d. making some people better fit to drive a sedan than a coupe

Explain what you know about the cause of mental illnesses.