Some cells in adult animals do not divide. They exit no G, phase and enter in an inactive stage called:

Options

A. `G_(2)` phase
B. `G_(0)` phase
C. S-phase
D. M-phase

Answer

### Step-by-Step Solution:

1. **Understanding Cell Division**: In adult animals, certain cells do not undergo division. This is an important aspect of cellular biology, especially in tissues that do not regenerate frequently.

2. **Identifying the Phase**: The question specifies that these cells exit the G1 phase of the cell cycle. The G1 phase is the first gap phase where the cell grows and prepares for DNA replication.

3. **Inactive Stage**: After exiting the G1 phase, these cells enter an inactive stage. This stage is characterized by a lack of cell division, even though the cells may still be metabolically active.

4. **Naming the Inactive Stage**: The inactive stage that cells enter after G1 is known as the **G0 phase** (also referred to as the quiescent phase). In this phase, cells are not actively preparing to divide but may still perform their normal functions.

5. **Examples of Cells in G0 Phase**: Heart cells (cardiomyocytes) and neurons are examples of cells that typically enter the G0 phase and do not divide in adult animals.

6. **Conclusion**: Therefore, the answer to the question is that the inactive stage where some cells in adult animals do not divide is called the **G0 phase**.