A

ACROSS 

I walk across Texas.

https://news.georgetown.org/files/2013/09/Walk-Across-Texas-logo.png

ADD   Just add one .

http://www.mealtime.org/uploadedImages/Mealtime/Home_Page/CFA_JustAddOne_FINAL(1).jpg

AMONG





The green apple is among the others apples.http://us.123rf.com/400wm/400/400/jannoon028/jannoon0281204/jannoon028120400124/13213339-green-apple-among-red-apples-isolated-on-a-white.jpg