1、some bat caves,like honeybee hives, have residents that take on different duties such as defending the entrance, （acting as sentinels and to sound） a warning at the approach of danger, and scouting outsides the cave for new food and roosting sites.
sounding a warning 是和前面acting as sentinels 平行的，同时acting as sentinels and sounding a warning at the approach of danger仅代表different duties 中的一个，并和前面的defending the entrance以及后面的scouting outsides the cave for new food and roosting sites平行。。。类似这样的结构在GMAT中还有很多，即大平行结构中包含小平行结构。
