The three correct answers that accurately describe coral reefs are:
- Coral reefs are sturdy structures that can exist for thousands of years.
- Coral reef ecosystems are among the most biodiverse on Earth.
- Coral reefs often are located in nutrient-poor waters.
(Note: The statement about coral reefs being located in nutrient-poor waters is correct; they thrive in clear, shallow waters with low nutrient levels, though they might sometimes be influenced by local nutrient sources.)