19. To help preserve ancient Egyptian monuments threatened by high water tables, a Swedish engineering firm has proposed installing pumps, perhaps solar powered, to lower the underground water level and dig trenches around the bases of the stone walls.

(A) to lower the underground water level and dig trenches

(B) to lower the underground water level and to dig trenches

(C) to lower the underground water level and digging trenches

(D) that lower the underground water level and that trenches be dug

(E) that lower the underground water level and trench digging

没人问过。

答案是C。后面的digging trenches是什么成分啊？