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

The correct answer is A. 1970-1980.

During the 1970s, there was significant investment in hydroelectric power as countries sought alternative energy sources, especially in response to the oil crises. Many large dams and hydroelectric plants were built during this decade, leading to a peak in hydroelectric power usage. If you have more questions or need further clarification, feel free to ask!