A:at dusk
B:at length
C:at present
D:at stake
①小麦种子的胚由胚芽、胚轴、胚根和子叶四个部分组成
②胚是种子的主要部分,是新植物体的幼体
③将一粒玉米种子纵切后,在切面滴碘液,整个切面都变得一样蓝
④红豆杉、黄皮、苏铁的种子外都没有果皮包被
The talk ____about twenty minutes.
-- What do you think of Bill?
-- I used to think that he ____ (be) an honest boy.