Correct Answer: Option B

Explanation

Downward delivery (also known as upward displacement of air) is a method used to collect gases that are denser than air. Air has an average relative molecular mass of approximately 29. 

CO (Carbon Monoxide) has a molecular mass of 28, making it slightly lighter than air. N\(_2\) (Nitrogen) has a molecular mass of 28, making it slightly lighter than air.
Because both gases in this pair are lighter than air, they will rise rather than settle at the bottom of a gas jar, meaning they cannot be collected by downward delivery.

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