The atoms in which X are equal are called isotones. Here, X is

Options

A. atomic number
B. mass number
C. electron
D. neutrons

Explanation

To solve the question, "The atoms in which X are equal are called isotones. Here, X is," we need to identify what X represents in the context of isotones.

### Step-by-Step Solution:

1. **Understanding Isotones**:
- Isotones are defined as atoms that have the same number of neutrons but different numbers of protons (and thus different atomic numbers).

2. **Identifying X**:
- Since isotones are characterized by having the same number of neutrons, we can conclude that X refers to the number of neutrons in the atoms.

3. **Conclusion**:
- Therefore, the correct answer is that X is equal to the number of neutrons.

### Final Answer:
X is the number of neutrons.

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