Sentence Correction

Unlike other arachnids, which have their nerve cells evenly distributed along their bodies, the scorpion's nerve cells are clustered in its head, like a mammal's.

(A) bodies, the scorpion's nerve cells are clustered in its head, like a mammal's
(B) bodies, the scorpion's head has a cluster of nerve cells, as a mammal does
(C) body, the scorpion has a cluster of nerve cells in its head, as a mammal does
(D) body, nerve cells are clustered in the scorpion's head, like a mammal's
(E) body, a cluster of nerve cells is in the scorpion's head, like a mammal's

Q/A-c why not a

This tests modifier errors. First, to make sense of the sentence, ignore everything from "which have" to "their bodies". The original sentence becomes:

Unlike other arachnids, the scorpion's nerve cells are clustered in its head, like a mammal's.

"Unlike x, y" is a direct comparison in which x and y must be comparable. In this case, answer (A) is incorrect because it compares the scorpion's cells to other arachnids. A better answer would compare the scorpion itself to other arachnids.

We can use this realization to get rid of B (compares "the scorpion's head" to "other arachnids"), D (same as A), and E (compares "a cluster of cells" to "other arachnids".

answer C is correct because it correctly compares the scorpion itself to other arachnids.

Hope this helps. My signature below has more info.
-Patrick