Read the following passage and write the letter A, B, C or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks.

Renewable energy sources are the forms of alternative energy that are being constantly renewed by the (31) ______ processes of the Earth, (32)______  the sunlight from the sun or the wind from the air, and are therefore environmentally friendly. Renewable energy has been used as a clean, renewable, and affordable energy source in one way or another for thousands of years. (33) ______ the world needs to replace fossil fuels with alternatives that are replenishable, (34)______  energy resources, even renewable energy sources are limited.

The use of renewable energy sources is definitely what we want to move (35)______ in the future, but their cost and reliability will admit how quickly (36)______ will grow. Wind and solar photovoltaic power generation has the most potential for growth since the cost of generating green electricity using these devices has decreased a lot in the past couple of decades. (37) ______, the (38)______  in weather conditions, geography (39) ______energy demand usually means that every type of energy storage in needed to (40)______ a constant supply.