Usually (A)____ are oxidised to release energy but (B)____ and even (C)____ can be used as respiratory substances in some plants.

Answer

To fill in the blanks in the question, we can follow these steps:

1. **Identify the common respiratory substances**: The question indicates that we need to identify the substances that are typically oxidized to release energy in plants. The most common respiratory substance in both plants and animals is carbohydrates.

2. **Fill in the first blank (A)**: Based on our identification, we can conclude that the first blank (A) should be filled with "carbohydrates."

**Answer for A**: Carbohydrates

3. **Identify alternative respiratory substances**: The question also mentions that there are other substances that can be used as respiratory substances in some plants. These include fats and proteins.

4. **Fill in the second blank (B)**: For the second blank (B), we can fill it with "fats," as they are commonly used as an alternative source of energy.

**Answer for B**: Fats

5. **Fill in the third blank (C)**: Finally, for the third blank (C), we can fill it with "organic acids," which can also serve as respiratory substances in certain plants.

**Answer for C**: Organic acids

Putting it all together, the complete sentence is:
"Usually (A) carbohydrates are oxidized to release energy but (B) fats and even (C) organic acids can be used as respiratory substances in some plants."