某患者上颌牙列可用间隙是83mm，必须间隙是79mm，其拥挤记录为（）

A.0mm

B.4mm

C.0～4mm

D.4mm以上

E.以上均不对

A chain bridle is used when towing astern because it（）.

A.is easy to connect

B.provides an effective catenary and absorbs shock due to its weight

C.makes rigging a swivel unnecessary

D.prevents the tow from yawing by the drag of the chains in a seaway