20. (5%) The cutoff frequency for the T'Mg mode in the air-filled parallel plates (with separation of 1 cm) will be somewhere in the frequency range of
(A) DC~20 GHz
(B) 20~40 GHz
(C) 40~60 GHz
(D) 60~80 GHz

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統計： A(1), B(0), C(0), D(0), E(0) #2853395