A:had been finished
B:were finished
C:having been finished
D:been finished
如图,平面ABCD⊥平面ABEF,四边形ABCD是正方形,四边形ABEF是矩形,且AF=12AD=a,G是EF的中点,则GB与平面AGC所成角的正弦值为( )
— _________ do you have an Art Festival in your school? — Once a year.
下面对文章内容的分析,不正确的一项是 ( )
一般情况下,下列的细胞中分化程度由高到低的顺序依次是( )①卵细胞 ②受精卵 ③胚胎干细胞 ④造血干细胞 ⑤神经细胞
生物圈‖的对照组是地球生物圈.