O is what should go in the blank. O stands for Oxygen.
The velocity of the gas is inversely proportional to the square root of the molar mass of the gass. Heavier molecules tend to travel slow compared to light molecules. Among these, He has the lowest molar mass of 4 g/mol hence this element has the greatest velocity among all molecules given.
B.) Because they both are Acids
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The first one is Solid, or A.
The second: B or liquids.
Third: also B I think.
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