Sources of gradual change in American public opinion include:

The correct answer is C. Cohort replacement, changing technology, and geographic mobility.

Explanation: Each of these factors significantly impacts public opinion over time. Cohort replacement refers to the phenomenon where younger generations replace older ones, bringing new perspectives and values. Changing technology affects how people access information and communicate, influencing their opinions. Geographic mobility results in demographic shifts which can alter community values and opinions as people relocate and mix with different populations.