Which of the following sentences requires an appropriate citation within a writing assignment?

According to billboards popping up throughout Hanlon County, vitamin C is the best way to prevent the common cold.

According to a recent study published in the Midwestern Journal of Homeopathy, taking vitamin C alone is not enough to prevent the common cold.

Many of us feel that taking extra vitamin C does not do enoygh to help prevent the common cold.

My doctor used to remind me to drink orange juice every day. She would exclaim, $"$It is so good for you and chock$-$full of vitamin C$!"$