Give an example of a plant which came into India as a contaminant and is a cause of pollen allergy.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for an example of a plant that was introduced to India as a contaminant and is known to cause pollen allergies.

2. **Identifying Alien Species**: Alien species are those that are not native to a particular habitat and are introduced by human activities. These species can have unpredictable consequences on the local ecosystem.

3. **Recognizing the Impact of Alien Species**: When an alien species is introduced, it often lacks natural predators in the new environment. This allows it to multiply rapidly, leading to invasive growth that can threaten native species and biodiversity.

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