洋葱表皮细胞和人的口腔上皮细胞都具有的结构是( )
①细胞壁 ②细胞膜 ③叶绿体 ④线粒体 ⑤细胞核 ⑥液泡 ⑦细胞质
A:①②④⑤⑦
B:②③⑤⑦
C:②④⑤⑦
D:②④⑥⑦
中国历史有了确切纪年的时间是( )
A
The Double Ninth Festival is a traditional (传统的)Chinese festival. It usually comes in October or November. On this day,people always wear the zhuyu (茱萸)plant. They usually climb mountains. Sometimes they go to see flower shows. The Double Ninth Festival is a festival for old people. People usually visit their parents and grandparents on this day. They often eat Double Ninth cakes together.
自然状况下,鸡有时会发生性反转,如母鸡逐渐变为公鸡。已知鸡的性别由性染色体决定。如果性反转公鸡与正常母鸡交配,并产生后代,后代中母鸡与公鸡的比例是 ( )