A radioactive element `A` disintegrates in the following manner:
`A overset(- a)rarr B overset(-beta)rarr C overset(-beta)rarr D`
Which one (s) the elements `A, B, C`, and `D` are isotope (s) and which one (s) `is//are` isobar(s)?`

Explanation

To solve the problem, we need to analyze the decay process of the radioactive element A and determine the relationships between the elements A, B, C, and D in terms of isotopes and isobars.

### Step-by-Step Solution:

1. **Identify the Initial Element A:**
- Let the atomic number of element A be \( Z \) and the mass number be \( M \).
- Therefore, we can represent element A as \( \text{A} \) with \( \text{A} \) having \( Z \) protons and \( M \) nucleons (protons + neutrons).

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