The greenhouse effect is a natural phenomenon.Without a natural greenhouse effect,the temperature of the earth would be about -18°C instead of its present 14°C.Why are scientists concerned about the greenhouse effect?

A.There is no technology that will keep the temperature of earth constant.
B.Planting trees in wider areas is not possible;earth has many desert areas.
C.Human activities like burning of fossil fuels and deforestation enhance the greenhouse effect.
D.The huuman population is increasing rapidly;more carbon dioxide will be released to the atmosphere.​