hers

"Ducks and sparrows are birds."

"My sister is a female student in her senior year of high school. "

"My sister brought several books to the library, and that one is about dinosaurs. "

"This book is my favorite. "

"Each person is unique, both mentally and physically."

"Where does this document belong?"

"Follow that car!"

"My brother's careful think about his future career helped him choose a good college. "

"The possessive case of the word "student" is "student's". "

"By now, you must surely know which is which."

"A cat is an animal, not a plant. Humans are also animals, under the scientific definition, as we are not plants."

"The school is located just outside the town, without the main shopping area. "