Alarge number of art songs were written and displaying a wide variety of styles. which common element do they share? length poetry language key

Of all the common elements from the four choices from the large number of art songs that were written and displayed a wide variety of styles, length is the only thing that they have in common. The other three choices are poetry, language, and key. 

The answer is a : varying the size of objects or subjects, so that some are smaller than others and the illusion of depth is created.hope this : )