One of the Problems with Single Rating Scale Measures of Speech

One of the problems with single rating scale measures of speech intelligibility is:

A) that listeners may not understand the speaker adequately enough to rate a child's speech intelligibility.
B) the interdependence of the listener and the speaker, and therefore that listeners will make different judgments.
C) they provide information about functional communicative success.
D) that listeners need extensive training to complete intelligibility rating scales.