..................... is the technique by which the three-dimensional structures of macromolecules can be studied.

Explanation

**Step-by-Step Solution:**

1. **Identify the Subject Matter**: The question asks for a technique used to study the three-dimensional structures of macromolecules, which include proteins, carbohydrates, and nucleic acids.

2. **Understand Macromolecules**: Macromolecules are large complex molecules that are essential for life. The main types include proteins, nucleic acids (like DNA and RNA), and carbohydrates.

3. **Recognize the Techniques**: The transcript mentions two specific techniques for studying the 3D structures of macromolecules:
- **X-ray Crystallography**: This technique involves crystallizing the macromolecule and then using X-ray diffraction to determine the arrangement of atoms within the crystal, allowing for the construction of a 3D model.
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