The phrase this source in the passage refers to

D

Cụm từ “ this source” trong bài đọc ám chỉ đến.........................

Sun: mặt trời

Wind: gió

Rivers: sông

Oceans: đại dương

Dẫn chứng: The oceans, too, represent an impressive source of potential energy. For example, it has been estimated that the oceans could provide nearly 3,000 times the energy generated by hydroelectric dams such as the Hoover Dam. Yet, this source remains quite difficult to exploit.

=>Chọn D