Distinct O2 molecular masses from isotope combinations

Steps:

• Oxygen isotopes are ^{16}O, ^{17}O, ^{18}O (the third stable isotope is ^{17}O).
• Possible O_2 pairs: ^{16}O-^{16}O (mass 32), ^{16}O-^{17}O (33), ^{16}O-^{18}O (34), ^{17}O-^{17}O (34), ^{17}O-^{18}O (35), ^{18}O-^{18}O (36).
• Unique masses are 32, 33, 34, 35, 36 (34 from two combinations but one peak).
• Five distinct masses yield five peaks in mass spectrometry.

Why B is correct:

• Mass spectrometry detects peaks at unique mass-to-charge ratios; five different O_2 masses produce five peaks.

Why the others are wrong:

• A: Counts only homonuclear isotopes, ignoring mixed pairs.
• C: Treats mass 34 combinations as separate peaks, but they overlap.
• D: Assumes nine unique combinations without merging identical masses.

Final answer: B