Final answer:
A cold frame greenhouse is usually covered with a transparent material, such as glass or clear plastic, to allow sunlight to enter and trap heat inside.
Step-by-step explanation:
A cold frame greenhouse is usually covered with a transparent material, such as glass or clear plastic, to allow sunlight to enter and trap heat inside.
This covering helps create a warm and protected environment for plants, extending the growing season and protecting them from harsh weather conditions.
The transparent cover also allows for the passage of visible light, which is essential for photosynthesis, the process by which plants convert sunlight into energy.