Question: Why plants do not utilize nitrogen directly from atmosphere? Ans. Most of the plants cannot utilise nitrogen directly from the atmosphere. They use nitrogen only when it is converted into nitrates and nitrites by the process nitrogen fixation. Question: Name the substance that reduces the amount of dissolved oxygen in water. Ans. Excessive growth of phytoplanktons reduces dissolved oxygen level in water Question: Name the organisms found to be very sensitive to the levels of contaminants like sulphur dioxide in the air. Ans. Lichens Question: What do you understand by the term ‘Natural resources’? Ans. Natural resources are the variety of substances that human beings get from the nature to meet their basic needs. Question: What is strip-cropping? Ans. Strip-cropping is the planting of crops in rows or strips to check flow of water. It is practiced in the sloping areas of hills to check soil erosion Question: What is greenhouse effect? How is it caused? Ans. Greenho
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