题干

动物的卵细胞的形成与精子形成过程的不同点是
①次级卵母细胞将进行普通的有丝分裂   ②一个卵原细胞最终分裂只形成一个卵细胞    ③一个卵原细胞经复制后形成一个初级卵母细胞   ④卵细胞不经过变形阶段  ⑤一个初级卵母细胞分裂成的两个细胞大小不等 ⑥卵细胞中的染色体数目是初级卵母细胞的一半

A:②④⑤

B:①③⑤

C:②③⑥

D:①④⑥高

上一题 下一题 0.0难度 选择题 更新时间:2010-07-22 10:01:26

答案(点此获取答案解析)

A

同类题4

任务型阅读

Cowboys in the US

    Cowboys played an important role in the settling of the west. Ranching (牧场经营)was a big industry and cowboys helped to run the ranches.____

The Roundup

    Each spring and fall the cowboys would work on the “roundup”____ Cattle would walk freely much of the year and then the cowboys would need to bring them in. In order to tell what cattle belonged to their ranch, the cattle would have a special mark burned into them called a “ brand “.

Horse and Saddle (马鞍)

    The most important item of any cowboy was his horse and saddle. The saddles were often custom-made (定制的) and, next to his horse, probably the most valuable item a cowboy owned.____

Clothing

    Cowboys wore special clothing that helped them with their jobs. They wore large 10-gallon hats to protect them from the sun and the rain. They wore special cowboy boots that helped them to get in and out of the stirrups (马镫)when riding a horse____ Many cowboys wore chaps on the outsides of their legs to help protect them from sharp bushes that their horse may rub up against.

Cowboy Code

    ____ The code included such rules as being polite and respectful, always saying “howdy”, not waving at a man on a horse (you should nod) , never riding another man's horse without his permission,always helping someone in need, and never putting on another man's hat.

A. They herded cattle, repaired buildings, and took care of the horses.

B. The cowboys of the Old West had an unwritten code that they lived by.

C. This when the cowboys would bring in all the cattle from the open ranch.

D. Horses were so important that anyone caught stealing a horse was usually hanged!

E. Their clothing grew out of practical need and the environment in which they worked.

F. This was especially important if they fell, so they wouldn't get dragged by their horse.

G. They have deep historic roots dating back to the earliest European settlers of America.