In cold-water habitats, certain invertebrates and fish convert starches into complex carbohydrates called glycerols, in effect manufacturing its own antifreeze.
(A) in effect manufacturing its own antifreeze
(B) effectively manufacturing antifreeze of its own
(C) in effect manufacturing their own antifreeze.
(D) so that they manufacture their own antifreeze
(E) thus the manufacture of its own antifreeze

Y not D ,