GWD28-21 一个新问题

28-21: According to entomologists, single locusts are quiet creatures, but when locusts are placed with others of their species, they become excited, change color, vibrate, and even hum.

A: when locusts are placed with others of their species,

B: when placing them with others of their species, then

C: locusts, when placed with others of their species,

D: placing them with others of their species,

E: placed with others of their species, then

我想问一下，E选项的问题到底是什么：then只能连接两个并列结构吗？是否把then去掉就可以了？就变成：

...,but placed with others..., they become...

placed在这里做状语。

多谢！！