list of 25 biggest convenient chain stores in the US (like shoprite and kmart)

ShopRite is a supermarket.  Kmart is a discount store.  You may want
to enlighten the researchers on the specific type of store you want. 
Convenient chain store is too general.

Perhaps you may want to up your price.
Sounds like a lot of research to me for a measly two bucks.

My 2 cents,

Steph53

Both of the comments above make excellent points. 

Another thing that occurs to me is that you need to define what you
mean by "biggest." Do you mean "chains with the greatest number of
stores" or "chains that earn the largest amount of money," or
something else?