提货单表明了持有人有权得到该提货单所标明物品的权利，说明提货单是一种（）。

A.有价证券

B.商品证券

C.货币证券

D.资本证券

They were walking along the shore _______ a huge wave appeared out of nowhere, sweeping them out to sea.[ ]

A．as  

B．while

C．when  

D．though