Back-up wires on a towed vessel provide（）.

A.a factor of safety

B.additional strength

C.a distribution of the towing load

D.All of the above

根治性颈清扫术包括切除胸锁乳突肌、颈内静脉、副神经、迷走神经在内的所有I—V区颈部淋巴结。