Al must build 25 radios before he receives $20. What schedule of reinforcement is being used?
A. a variable-ratio schedule
B. a fixed-ratio schedule
C. a fixed-interval schedule
D. a continuous schedule

B. a fixed-ratio schedule                                      

Explanation:

Fixed-ratio schedule: In psychology, the term "fixed-ratio schedule" is considered as one of the reinforcement schedules in operant conditioning in which a specific response is being reinforced once particular responses are being completed.

The different reinforcement schedules was proposed by one of the famous behaviorists and psychologists named B. F. Skinner.

In the question above, the given statement represents the "fixed-ratio schedule".

Olmec civilization

Explanation:

The Olmec civilization was the first Mesoamerican civilization, beginning around 1600–1400 BC and ending around 400 BC. Mesoamerica is considered one of the six sites around the globe in which civilization developed independently and indigenously.