持续性葡萄胎是指葡萄胎排空后下列多长时间,血清hCG持续阳性
A.12周B.4周C.20周D.8周E.16周
参考答案:A
A ______ consists of the symbols, characters, and usage roles that permit people to communicate with computer.
A.programming language
B.network
C.keyboard
D.display
材料相同、横截面面积相等的两拉杆,一为圆形截面,另一为方形截面,在相同拉力作用下,两拉杆横截面上的应力()