yes, i agree....answer has to be C
i'll keep the explanation simple:
A, B and E are incorrect b/c they use "its"....we are talking about invertebrates and fish, thus we must use "their" ....there are other reasons why these choices are wrong, but this is the simplest way to eliminate them...
This is tough to explain, but i'll try:
D is incorrect b/c the way the non-underlined part of the sentence is written. That part of the sentence is describing something (a process) and the remainder of the sentence is likening that process to something (which is the underlined part of the sentence)....therefore it would not make sense to use "so" after the comma b/c that would effectively say that the invertebrates and fish ARE
really manufacturing their own antifreeze...i've never seen antifreeze coming out of fish, have you?
plus, D just simply "sounds" wrong, but i don't like to say that b/c non-native english speakers can not pick up on that...