13. 下列哪一個選項的敘述是在定義此樂曲之曲種？ (A)a fixed verse form consisting usually of three stanzas with recurrent rhymes, an envoi, and an identical refrain for each part. (B)a traditional folk song sung by Venetian gondoliers, or a piece of music composed in that style. (C)a genre of chamber music that was quite popular in the Classical era. (D)a musical composition that reflects the moods and feelings of night time.