Using IF for data validation

Would like to seek your assistance. I trying to make a condition using data validation. this is the scenario:
for column C8 (this is the dropdown list Customer: A, B, C D). Each customer has diff products. I created Name manager for each customer (e.g. AItems - for A customer; BItems; CItems and DItems.
I listed in column AA and AB the details:
column AA Column AB
A Popcorn
A Coke
A Cupcake
A Candy
B Burger
B Fries
C Coffee
C Tea
D Rum
D Beer

How can I use IF statement for this? I tried this condition but didnt work. =IF($C$8="A",AItem,BItems,CItems,DItems)
Appreciate your recommendation.