胚胎第5周时主要造血器官是
A.卵黄囊B.胆囊C.淋巴管D.肝脾E.骨髓
参考答案:A
解析:胎儿血循环约于受精后3周末建立,其红细胞主要来自卵黄囊。
The pictures __the visitor took at the river bank __ to the students.
A.that; have given
B.which; given
C.that; have been given
D.which; would be given
连续启动不超过2次,每次之间的间隔应为()
A、5-8s
B、10-15s
C、8-10s
D、15-20s