站在物流活动承担主体的角度看，对于专业物流企业，竞争来自于（）。
A．行业内部
B．企业内部
C．货主企业
D．企业外部
参考答案：A, C
They _____ with each other at first sight and got married soon. Until now they _____ still _____ and live a
happy life. [ ]
A. fell in love; fall; in love
B. were in love; are; in love
C. fell in love; are; in love
D. were in love; fall; in love
将两台性能相同的离心泵并联，是为了增加（）。
A.扬程
B.流量
C.功率
D.效率
