what's the definition of vowel(s)?

The English pure vowels can be classified according to

A. the position of the lips.
B. the height of the raised part of the tongue.
C. the dgree of obstruction to the flow.
D. the part of the tongue raised.

/i/ is a much more tense sound than /i:/.

A. 对
B. 错

/e/ is a central vowel.

A. 对
B. 错

There are 20 pure vowels, in English.

A. 对
B. 错