Each of the following groups of sounds consists of the members of a natural class of sounds plus one sound that is not a member of that class. Identify the sound that does not belong to the class; name the feature or features that define the class (0.5 * 4 = 2.0).
[p],[d],[g],[b]
[a],[u],[i],[w]
[n],[f],[v],[b],[p]
[t],[m],[n],[?]