Which one of the following elements was not easily spotted by ancient cultures?

• A. Au (gold)
• B. O (oxygen)
• C. Ag (silver)
• D. Cu (copper)

Answer: (B)

Verified step by step guidance

1. Understand the question is about the historical discovery and recognition of elements by ancient cultures.
2. Recall that elements like Au (gold), Ag (silver), and Cu (copper) are metals that occur naturally in their native (metallic) form and were easily found and used by ancient civilizations.
3. Recognize that oxygen (O) is a gas and does not exist in a free, elemental form that can be easily collected or seen; it is mostly found combined in compounds like water (H\_2O) and oxides.
4. Conclude that because oxygen is not found as a free element and is invisible, it was not easily spotted or identified by ancient cultures compared to metals like gold, silver, and copper.
5. Therefore, the element that was not easily spotted by ancient cultures is oxygen (O).