Weihe river is the biggest river and principal surface water source in Guanzhong Basin.

渭河是关中地区最大的河流，是该地区主要的地表水水源。

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The integrated system is deployed on U.S. Navy MH-60R multi-mission helicopters whose principal mission is to protect the fleet from submarines and surface vessels.

该系统部署在美国海军的MH-60R多任务直升机上，其主要任务是保护港口的潜艇和水面舰艇。

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The principal mission of the MH-60R multi-mission helicopter is to protect the fleet from submarines and surface vessels and provide a maritime strike capability.

MH - 60r多任务直升机的主要任务是保护潜艇和水面舰队，并提供海上打击能力。

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It says the Chinese navy possesses about 75 principal surface combatants, like destroyers and frigates, and around 60 submarines.

WSJ: China Adds Aircraft Carrier to Its Navy

Professor John Zarnecki, principal investigator on Huygens' surface science package (SSP) said the area where Huygens landed appeared to have a thin crust that overlies a material with more uniform consistency something like wet sand.

BBC: NEWS | Science/Nature | Titan mission 'resounding success'

It is on that surface of the ice that they have adapted ways of catching seals that are their principal prey.

CNN: Polar bear now listed as 'threatened' species