Which of the following statement is true about air?
Options:
A .  
It is opaque.
B .  
It is made up of only one substance.
C .  
It contains water vapour.
D .  
It contains 70% of oxygen.
Answer: Option C : C Air is a mixture of invisible gases, that surrounds the earth. It mainly consists of gases namely nitrogen (78%), oxygen (21%), argon (0.9%), carbon-dioxide (0.03%) and water vapour. Composition of air is as follows:
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