前者.
/u:/的口型很紧,张开很小（上下嘴唇相距1厘米）,双唇向前撅成一个小圆圈,与汉字“驴”的韵母口型很像.要是按照汉字“鲁”的韵母发就错了.
To make the /u:/ sound,keep the mouth slightly open and the lips 1 cm apart.The lips are tense,and pushed forward into a small circle.
像/ju:/是错觉.