Nowhere in nature is aluminium (铝) found free, owing to (由于) its always  ____with other

elements (元素) most commonly with oxygen.  [ ]

A. combined    

B. having combined 

C. combine    

D. being combined

定时定量配送的缺点是( )。

A．配送模式计划性强，对操作准确性要求高

B．配送模式灵活性高

C．配送成本高

D．计划性差