PL HELP M The first music machines were those that we have all seen music boxes with pins that we could wind with keys. Then came larger machines with metal discs. These also came in boxes. Finally, someone developed the machines with paper rolls that could play the music of several instruments. We called them player pianos.

What is wrong with the underlined sentence?
A.
It needs a subject.
B.
It is too long.
C.
It is a run-on sentence.
D.
It is a sentence fragment.