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For a closed organ pipe of length $L$, which set of harmonics is allowed? 
AAll harmonics: $n=1,2,3,4,\dots$
BOnly even harmonics
COnly odd harmonics
DNo harmonics above the fundamental
Answer & Solution
Correct answer: C. Only odd harmonics
A closed organ pipe has a node at the closed end and an antinode at the open end. This boundary condition is satisfied only for lengths equal to odd multiples of $\lambda/4$, so only odd harmonics occur: 1st, 3rd, 5th, and so on. Therefore C is correct.
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