Which of the Following Describes the Northern Cities Shift

Which of the following describes the Northern Cities Shift?

A) A change in the vowel system of Standard British English during the 15th century which allowed it to take on more nonstandard features
B) A shift in the vowel system of northern, urban areas that is contributing to the finding that American regional dialects are becoming less alike
C) A shift in perception that explains how Standard American English was originally placed in New England but is now associated with speech in the Midwest
D) A shift in AAE from a dialect that was not written to one in which there is an emerging canon of literature
E) A historical phonetic change in which all stop consonants have shifted in their manner of articulation