脐带中含有的血管是()
A.1条脐动脉2条脐静脉
B.2条脐静脉2条脐动脉
C.1条脐静脉3条脐动脉
D.1条脐动脉和1条脐静脉
E.2条脐动脉1条脐静脉
参考答案:E
— Mum, I will perform at the opening ceremony of the Youth Olympic.
— ! We wish you a wonderful performance.
A.Thank you
B.The same to you
C.Congratulations
D.Never mind
在什么情况宜采用垂直剖面法、垂线法、数字标高投影法留设保护煤柱?