库房门与地面的缝隙不应大于（），且应采用金属门或下缘包铁皮的木门。
A.3mm
B.4mm
C.5mm
D.6mm
参考答案：C
共轭效应
A．离子交换色谱B．聚酰胺色谱C．葡聚糖凝胶GD．大孔吸附树脂E．硅胶或氧化铝吸附色谱选择合适的色谱条件分离下列各组化合物
19．The Internet and the effect______________brought about did a lot of good to our daily life and work.
A．that
B．what
C．which
D．It
