A:染色体已经分裂,但纺锤体形成受阻
B:未受精的卵细胞发育而成
C:染色体结构发生变化
D:花药离体培养的结果
Food is very important to our health. Everyone needs to eat well if he or she wants to have a strong body.Our minds, like our bodies, always need the best food. This kind of food is knowledge. We start getting knowledge when we are very young.
下图中ACB为晨昏线,C为中点。图示范围内阴影区为黑夜,其他地区为白天。此时与北京处于同一天的范围占全球的比例为 ( )
如图,点E是▱ABCD的边AD的中点,连接CE交BD于点F,如果S△DEF=a,那么S△BCF= ____.