locusts

[AnjaliOberoi]

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

OA A, what's the issue with [spoiler]C[/spoiler]

[theCodeToGMAT]

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,

C. locusts, when placed with others of their species,
INCORRECT; the underlined part introduces contradiction.. so we need "but"

[AnjaliOberoi]

Sorry, by mistake, i wrote "but" in the underlined part...

[theCodeToGMAT]

Using {C} in sentence

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

If we remove the part of the sentence in Blue, it becomes:

According to entomologists, single locusts are quiet creatures, but locusts they become excited, change color, vibrate, and even hum.

"they" is unnecessary.