( It ( ( is ( ( most efficient ) ( at ( radio ( wave frequencies ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (ADJP (ADJP (RBS most) (JJ efficient)) (PP (IN at) (NP (NN radio) (NN wave) (NNS frequencies))))) (. .)))
It is most efficient at radio wave frequencies.
(ROOT (S (NP (NNP Am) (NN radio) (NNS waves)) (VP (VBP exist) (PRT (RP in)) (NP (DT the) (ADJP (QP (CD 540) (TO to) (CD 1600))) (NN khz) (NN frequency) (NN range))) (. .)))