若一个数列的前n项和Sn=1-2+3-4+…+（-1）n+1n，则S17+S33+S50=（ ）。
1
If you go for a long ride in a friend's car, it's the custom to offer to ______ some of the expenses. [ ]
A. pay
B. pay off
C. pay for
D. pay back
急性普通型病毒性肝炎肝细胞病变为（）
A.气球样变
B.碎片状坏死
C.大块坏死
D.嗜酸性变
E.嗜酸性小体
