Isotopic combinations in O2 produce distinct molecular masses

Steps:

• Oxygen isotopes are 16O, 17O, 18O (three stable ones, despite question's wording).
• Possible O2 molecules: 16O-16O (mass 32), 16O-17O (33), 16O-18O (34), 17O-17O (34), 17O-18O (35), 18O-18O (36).
• Combinations yield masses 32, 33, 34 (from two types), 35, 36.
• Mass spectrometer separates by mass-to-charge ratio, giving 5 distinct peaks.

Why B is correct:

• Five unique masses (32–36) from isotopic pairs, per mass spectrometry principle of ion separation by m/z.

Why the others are wrong:

• A: Ignores mixed isotopes, assuming only three homonuclear peaks.
• C: Overcounts by splitting the two mass-34 combinations into separate peaks.
• D: Treats all six combinations as distinct, ignoring identical masses for 34.

Final answer: B