27. Which of the followings is true of vowels and diphthongs in North American English?
(A)Vowels are classified as high, mid, or low, referring to the level of the tongue with the oral cavity and the accompanyingraised or lowered position of the teeth.
(B)Vowels are characterized by the degree of tongue rounding or spreading that occurs during articulation.
(C)Vowels can be either tense or glided, referring to the amount of muscle tension used to pronounced the vowel.
(D)Vowels are classified as front, central, or back, depending on how far forward or back the tongue is positioned within theoral cavity during articulation.