求助一道英语单选,

题目

求助一道英语单选,
At last,the boy found themselves in a pleasant park with trees providing shade and__down to eat their picnic lunch
A.to sit B.sat C.sitting D having sat
为什么答案是B,不是C,sitting不应该是主动关系吗,如果B是因为是过去式的话,那为什么providing不用过去式?还有D选项有什么用法,

答案

是found and sat
而不是found……doing