The old man,together with his two dogs,___ walking ___ the river.

The old man,together with his two dogs,___ walking ___ the river.
The old man,together with his two dogs,___ walking ___ the river.
A.are,along
B.is,along
C.are,alone
D.is,alone
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我觉得应该是C或D,但是到底视为单数（整体）还是复数?