Why is CNG called an ecofriendly fuel?

Explanation

### Step-by-Step Solution:

1. **Definition of CNG**:
- CNG stands for Compressed Natural Gas. It is a type of fossil fuel that is primarily composed of methane.

2. **By-products of Combustion**:
- When CNG is burned, the primary by-product of its combustion is water vapor. This is in contrast to other fuels like petrol and diesel, which produce harmful by-products.

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