求问OG分册SC第2题~

蓝色大门

正确选项为：such as defending the entrance, acting as sentinels and sounding a warning at the approach of danger, and scouting outside the cave...
根据多个元素平行时and的使用规则，本题中第2个平行元素中不该有and吧。。。应该用逗号隔开变成4个元素平行，最后一个平行元素前有一个and
很是困惑，望大侠们指点

DUKB24

http://www.beatthegmat.com/the-bats-sentence-correction-t108281.html

look for ron's explanation


hope it will help