The main cause of geometrical isomerism is the ... carbon atoms about a ......... bond.

Explanation

To solve the question, we need to understand the concept of geometrical isomerism and identify the key components involved.

### Step-by-Step Solution:

1. **Understanding Geometrical Isomerism**:
- Geometrical isomerism occurs in compounds where the arrangement of atoms differs in space due to the restricted rotation around a bond. This is commonly seen in alkenes where there is a double bond.

2. **Identifying the Key Components**:
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