Comparing phase transition points of nitrogen, oxygen, and argon

Steps:

• Identify melting (mp) and boiling points (bp): N₂ (mp -210°C, bp -196°C), O₂ (mp -219°C, bp -183°C), Ar (mp -189°C, bp -186°C).
• Determine states at each temperature: solid if T < mp, liquid if mp ≤ T < bp, gas if T ≥ bp.
• Check for one element solid, one liquid, one gas.
• Evaluate choices: only -190°C satisfies all three states.

Why C is correct:

• At -190°C, Ar is solid (T < mp), O₂ is liquid (mp < T < bp), N₂ is gas (T > bp), per phase transition definitions.

Why the others are wrong:

• A: All solid (T < all mp).
• B: Ar solid, N₂ and O₂ liquid (two same states).
• D: All gas (T > all bp).

Final answer: C