Hey lheiannie07,

This question is all about number agreement. The first thing we should clue in on is that the answer choices switch between "are rarely fatal" and "is rarely fatal". The subject of the verb is "bite", which is singular, so the verb should also be singular. So we can eliminate all answer choices that use "are", which is plural. (Note: careful that you don't assume the plural "spiders" is the subject of the verb - while it is closer to "are/is", "of brown recluse spiders" only serves to describe "bite", making "bite" the subject.) This eliminates A and B.

After the comma is a switch between "they cause" and "it causes". Again, this refers to "bite", not "spiders". We have a singular subject, so we need singular pronouns and verbs. So we can eliminate all answer choices with "they cause". This eliminates A, B, and E.

Finally, we should notice the switch between "its" and "their" at the end of the underlined portion. Once again, our subject is singular, so we want a singular pronoun. We can eliminate all answer choices with "their". This eliminates B and C.

So we had 3 different agreement issues in this sentence! The key here is to be totally sure what subject your verbs/pronouns refer to - don't let descriptive phrases throw you off!