Unlike most Red Grinder species, the male and female speckled Red Grinder are very difficult to tell apart.

A. Unlike most Red Grinder species, the male and female speckled Red Grinder are very difficult to tell apart.
B. Unlike most Red Grinder species, the gender of the speckled Red Grinder is very difficult to distinguish.
C. Unlike those in most Red Grinder species, the male and female speckled Red Grinders are very difficult to distinguish.
D. It is very difficult, unlike in most Grinder species, to tell the male and female speckled Red Grinder apart.
E. Speckled Red Grinders are unlike most species of Red Grinder in that it is very difficult to tell the male and female apart.

IMO: C