Answer: Store C is a better buy
Step-by-step explanation: To find the better buy or cheaper cost for a shirt then we'll need to find the unit rate. Or look for the price for 1 shirt in each store. Divide the cost to the shirts given to get the cost of 1 shirt.
Store C: 2 shirts cost $12 --> $12/2= $6 for 1 shirt
Store D: 3 shirts cost $21 --> $21/3= $7 for 1 shirt