了400×了如÷45，
=七七00÷45，
=了000（只）
答：鹅有了000只．
In our school students are _____ according to how much they know and how old they are. [ ]
A. classified
B. combined
C. condemned
D. betrayed
描述某市、某年、某急性传染病发生水平的最佳指标是（）
A.患病率
B.感染率
C.罹患率
D.发病率
E.死亡率
