五跨关节中间跨为过渡跨，过渡跨两接触线屋脊处导线高度应高出正常高度40mm，允许偏差为±10mm。
参考答案：对
. As a teacher, he has never given his students so difficult a problem ____ they can’t work out.
A．if
B．that
C．when
D．As
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