A .  The lowest region of atmosphere is known as Troposphere.
B .  CO gas forms highly stable complex with blood known as carboxyhaemoglobin.
C .  Water vapor present in the atmosphere acts as greenhouse gas.
D .  Rain having pH value less than 7 is known as acid rain.
Answer: Option D : D a) The lowest region of atmosphere is known as troposphere and above this region lies the stratosphere. b) CO gas combines with hemoglobin present in blood to form carboxyhaemoglobin which is about 300 times more stable than oxyhaemoglobin. It reduces the oxygen carrying capacity of blood. c) Atmospheric gases which are transparent to sunlight but not to heat radiation from earth surface are known as greenhouse gas. Water vapor acts as greenhouse gas. Hence d) Rain having pH value less than 5.6 is termed as acid rain.
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