队列研究中主要的偏倚有（）。
A.回忆偏倚
B.选择偏倚
C.混杂偏倚
D.失访偏倚
E.以上都不是
参考答案：B, C, D
—The woman biologist stayed in Africa studying wild animals for 13 years before she returned.
—Oh, dear! She _______ a lot of difficulties!
A．may go through
B．might go through
C．ought to have gone through
D．must have gone through
在高压电场的作用下，水颗粒发生碰撞聚结合并，水靠（）分离沉降到脱水器底部，流入集水室，径排水放出。
A、重量
B、重度
C、油水密度差
D、粘度
