根据课文内容填空。

     It's 1.____ afternoon. The students are 2.____ 3.____ their 4.____. Wang Bing usually surfs 5.____ 6.____, goes 7.____ and plays 8.____ at the 9.____. Mike often goes 10.____, 1istens to 11.____ or 12.____. Helen likes 13.____ 14.____ at home. Yang Ling often does 15.____ at the weekends. Su Hai and Su Yang often 16.____ 17.____ and sometimes they 18.____ 19.____ in the park.

与导致臀先露无关的因素是（）

A.羊水过少

B.羊水过多

C.胎头衔接受阻

D.前置胎盘

E.原发性宫缩乏力