Why are crop fields known as artificial ecosystems?

Which of the following statements is incorrect?

The most important compound which accounts for almost $80\%$ of the total depletion of the ozone layer is $.........$

Define ecosystem.

Which of the following is an environmental problem?

In the following groups of materials,which group$(s)$ contains only non-biodegradable items?
$(i)$ Wood,paper,leather
$(ii)$ Polythene,detergent,$PVC$
$(iii)$ Plastic,detergent,grass
$(iv)$ Plastic,bakelite,$DDT$