美国南北战争爆发的标志是[ ]
A．1860年林肯当选总统
B．1861年南方挑起内战
C．1861年北方挑起内战
D．1865年里士满战役
答案：A
She had just finished her homework _____ her mother asked her to practice playing the piano yesterday. [ ]
A. when
B. while
C. after
D. since
A．a为动滑轮，B端钢缆受到滑轮组的拉力大小约为3G
B．a为动滑轮，B端钢缆受到滑轮组的拉力大小约为G
C．a、c为动滑轮，B端钢缆受到滑轮组的拉力大小约为3G
D．a、c为动滑轮，B端钢缆受到滑轮组的拉力大小约为G
G
G