Help** 50 points**
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                                          London, UK 2016

The quiet dripping disturbed the otherwise silent alley. Caroline stepped to the side to avoid tripping over a man on the street. She opened the door at the end of the street, and stepped inside.

"You're late!" A man's voice said as he stepped from the shadows.

"Sorry, I lost track of time."

"Well, you've got a new case, so I'd stop being late if I were you."He said.

"Jack, I'm never late when I'm on a case."

Jack sighed and handed her an envelope. Caroline opened it and read through it.

"Lucinda Waseford, dead in her parlor room. Let's see, she was found by her sister, stabbed in the back." She read.

"Yup, and you've been hired to investigate!" Jack said, "Now get to it, the sister is expecting you at 2 o'clock."

Turning the corner of  Deals Gateway Av. Caroline looked up. Several historic buildings lined the street. 159 Deals Gateway Av. She walked up to a beautiful brick building, and rang the doorbell. A pretty blonde woman with a pleasant face opened the door.

"Oh...You must be the detective. Come on in." She said.

Caroline stepped through the white framed door, but as she stepped in, she noticed a small red mark on the frame.

"Wait a minute! What's this mark?"She cried.

She took a closer look  and a similar mark on the hand rail next to the door.  

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."