Read the following passage carefully, and then select the best option A, B, C or D to complete it

Conservation, sustainable use and protection of (1) _______ resources including plants, animals, mineral deposits, soils, clean (2) _______, clean air, and fossil fuels such as coal, petroleum, and natural gas. Natural (3) _______ are grouped into two categories, renewable and nonrenewable. A (4) _______ resource is one that may be replaced over time by natural processes, (5) _______ fish populations or natural vegetation, or is inexhaustible, such as (6) _______ energy. The goal of renewable resource conservation is to ensure (7) _______ such resources are not consumed faster than they are (8) _______. Nonrenewable resources are those in limited supply that cannot be replaced (9) ______ can be replaced only over extremely long (10) _______ of time. Nonrenewable resources include fossil fuels and mineral deposits, such as (11) _______ ore and gold ore. Conservation activities for nonrenewable resources focus (12) _______ maintaining an adequate supply of these resources well into the future.

Natural resources are conserved for their biological, economic, and recreational values, (13) _______ their natural beauty and importance to local cultures. (14) _______, tropical rain forests are protected for their important role in both global ecology and the economic livelihood of the local culture; a coral reef may be (15) _______ for its recreational value for scuba divers; and a scenic river may be protected for its natural beauty....

Question 21:

Choose the word whose underlined part has a different pronunciation from the others in each group must natural