为了保证电磁铁可靠吸合，电磁吸力特性与反力特性必须配合好（）
参考答案：对
It came as_______ surprise that Poland’s President and scores of its top officials were killed in _______ air crash in Russia.
A．不填; the
B．a; an
C．a; 不填
D．the; an
接触线距轨面的高度在隧道内困难情况下不小于（）mm。
A.5700
B.5650
C.5600
D.5250
