理论数就是假定各总体率相同时计算出的各类观察数。
参考答案：对
煤矿工人常见肺尘埃沉着病为
A．硅沉着病
B．煤矽肺
C．煤肺
D．硅酸盐肺
E．混合肺尘埃沉着病
Mary can play the chess _____ she can't swim. [ ]
A. and
B. or
C. but
D. so
