Mr Smith, together with his wife and children,____in Shanghai 10 days ago .

C
这种复合主语取决于第一个主语，就是mr smith，根据时间状语 10 days ago，显然是过去式，所以选C was

A
with his wife and children是伴随状语，主要修饰Mr Smith

is，together with在句中，谓语单复数与主语保持一致

UU

选C，选A的人都没有注意10 days ago，要用过去时态。

选A。因为Mr.smith后面加了个逗号，后面的together with…是用来补充他的。