we have specials ( ) hamburger at KFC A/in B/for C/on D/at

we have specials ( ) hamburger at KFC A/in B/for C/on D/at
we have specials ( ) hamburger at KFC
A/in B/for C/on D/at
答案上给的是A/in,为什么是in,不是for? 望详细解答!谢谢.