3. The correct answer is "Deforestation prevents desertification".
Deforestation is the clearing of large area by cutting trees which belonged to a forest. Desertification means becoming a desert. Deforestation is also a reason which contributes to the desertification. Other than desertification, deforestation causes for drying out of soil, decreasing of biodiversity and loss of soil stability.
4. The correct answer is "an algal bloom".
Due to the dumping of chemical waste into the stream the algal species can grow up in the stream. The sudden increase in algae is called as an algal bloom. This causes to the
decrease of dissolved oxygen level of the stream.
