How many elements are presents in the d-block of the periodic table ?

Explanation

To determine how many elements are present in the d-block of the periodic table, we can follow these steps:

### Step 1: Understand the d-block
The d-block elements are transition metals found in groups 3 to 12 of the periodic table. They are characterized by the filling of d orbitals.

### Step 2: Identify the series
The d-block consists of four series based on the principal quantum number (n):
- 3d series (n=3)
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