All of the following are symbols we see in the text in chapters 15-19 except:

a card

a skull

a lightbulb

the desert

stargirl

the krebs cycle and electron transport occur in the mitochondria, though other than that i would need to know more information.

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."