Which toxic element or compound is released by vehicles?

Explanation

### Step-by-Step Solution:

1. **Identify the Source**: The question asks about the toxic elements or compounds released by vehicles. Vehicles, particularly motor vehicles, are the primary focus.

2. **Understand Emissions**: Motor vehicles emit various gases and pollutants as a result of fuel combustion. The most significant emission from vehicles is carbon dioxide (CO2).

3. **Recognize Other Pollutants**: In addition to carbon dioxide, vehicles also emit other harmful pollutants. These can include nitrogen oxides (NOx), carbon monoxide (CO), particulate matter (PM), and hydrocarbons (HC).

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