The first transition series is called __________transition series.

Explanation

To answer the question, "The first transition series is called __________ transition series," we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Transition Series**:
- The transition series refers to the elements in the d-block of the periodic table. These elements are characterized by the filling of d orbitals.

2. **Focus on the First Transition Series**:
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