In what time period did hydroelectric power usage reach its peak?

The correct answer is A. 1970-1980.

During this time period, many countries heavily invested in hydroelectric projects, resulting in the development of significant hydroelectric capacity. This period saw an increase in hydroelectric power usage as it became a preferred source of renewable energy due to its ability to generate large amounts of electricity and its relatively low operational costs once established. If you have more questions or need further clarification, feel free to ask!