Explain why the study of the human body can be overwhelming

There is so much to learn about, and it is hard to explain most of it, because we don't know why are bodies does some of the things it does.

i think its d, but grammarly says none

explanation:

the pronoun reference replaces a noun, but you have to make sure there's no room for confusion.

in the first sentence "her" could be referring to shely or to the sister. the answer there is "shelly called her sister and told her about the new baby."

in the second sentence "it" could be referring to the roof or the floor. the answer there is "since the roof leaks on the family room floor, we need to repair the roof."