He is one of these men who I am sure,always do ____ best even in most trying circumstancesa,his b.their为什么选B 主语怎么判断?

He is one of these men who I am sure,always do ____ best even in most trying circumstancesa,his b.their为什么选B 主语怎么判断?
He is one of these men who I am sure,always do ____ best even in most trying circumstances
a,his b.their
为什么选B 主语怎么判断?

He is one of these men who I am sure,always do ____ best even in most trying circumstancesa,his b.their为什么选B 主语怎么判断?
翻译过来：
他是我比较信任的那一类人,这种人通常尽他们最大努力去做事情.
所以后面的主语是他们,要用their了.

do your best
do their best
do my best