Air

Answer: C

N₂ 78%.

Trap A Uses everyday or macroscopic thinking instead of particles: Ask what the particles, ions or electrons are doing. Properties of a substance come from its particles and bonds, not from how it looks or feels.

Trap B Takes one factor as the whole answer: Ask whether the option is necessary or sufficient. If the stem involves two changes or two conditions, both must appear in the answer.

Trap D Miscounts atoms, electrons or steps: After working, check by counting back: total electrons = atomic number; atoms in the formula match the subscripts; number of half-lives = time ÷ half-life.