How many types of heavy water are possible? Write down formulae of all possible heavy water molecules.

Explanation

To determine how many types of heavy water are possible and to write down the formulae of all possible heavy water molecules, we need to understand the concept of heavy water and its isotopes.

### Step-by-Step Solution:

1. **Understanding Heavy Water**:
- Heavy water is water that contains deuterium (D), an isotope of hydrogen, instead of the common hydrogen (H). The presence of deuterium gives heavy water different properties compared to ordinary water (H₂O).

2. **Identifying the Isotopes**:
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