Um, I need a 3D.

I’m a bright cookie:
Me: “Excuse me, I need help with the batteries.”
Saleswoman: “Yes?”
Me: “I just bought a doorbell but I forgot the battery. The package says I need a 3-D battery, but I only see 4D, 7D, and 8D.”
Saleswoman: “Um, the number stands for the number of batteries in the package dear.”
Me: “Oooooooh.”