Complete the sentences below: 
Copper, silver, and gold appear in nature in their ________, therefore, is known for thousands of years.  However, most compounds are difficult to find in nature due to them _______.


being easily oxidized and thus being covered with colored film
natural state 
elemental form
being underground 
readily forming compounds

• A. Copper, silver, and gold appear in nature in their natural state, therefore, is known for thousands of years. However, most compounds are difficult to find in nature due to them being easily oxidized and thus being covered with colored film.
• B. Copper, silver, and gold appear in nature in their natural state, therefore, is known for thousands of years. However, most compounds are difficult to find in nature due to them being underground.
• C. Copper, silver, and gold appear in nature in their elemental form, therefore, is known for thousands of years. However, most compounds are difficult to find in nature due to them being easily oxidized and thus being covered with colored film
• D. Copper, silver, and gold appear in nature in their elemental form, therefore, is known for thousands of years. However, most compounds are difficult to find in nature due to them readily forming compounds.

Answer: (D)