The section presents a question related to chemistry, specifically about the main component of iron oxide. The key topics and entities include:

- **Problem Number**: 4
- **Problem ID**: KHTN7C1B2_L2_4
- **Main Question**: Identifying the primary element in iron oxide.
- **Answer Choices**:
  - A. \( 0 x y \) (not relevant)
  - B. Iron (Sắt)
  - C. Calcium (Canxi)
  - D. Carbon (Carbon)

Summary: The section poses a chemistry question regarding the primary component of iron oxide, with answer options including iron, calcium, and carbon.