反复发生的同类事故，并不一定产生完全相同的后果，这就是事故损失的()。
A.偶然性
B.必然性
C.相关性
D.因果关系
参考答案：A
The teacher and writer, as well as many guests ____ invited to the party held yesterday.[ ]
A. is
B. were
C. are
D. was
多普勒超声是指
A.振幅调制型
B.辉度调制型
C.彩色血流显像
D.多普勒频谱显示
E.以上都不是
