Final answer:
Benzodiazepines & Barbiturates are classes of substances known for their depressant effects on the central nervous system, including drugs like Valium and phenobarbital.
Step-by-step explanation:
The class of substances known for their depressant effects on the central nervous system and includes drugs like Valium and phenobarbital is a. Benzodiazepines & Barbiturates. Benzodiazepines and barbiturates are both central nervous system depressants that tend to suppress brain activity. They work by enhancing the activity of the neurotransmitter gamma-Aminobutyric acid (GABA), which has a calming effect on the brain.