ANSWER
D. The grocery store charges $1 less per lightbulb.
Step-by-step explanation
To find the cost per lightbulb we have to divide the total cost of each pack by the number of lightbulbs in each pack. For the hardware store,
The cost per lightbulb is $5. And for the grocery store,
The cost per lightbulb is $4.
Hence, the grocery store charges $1 less per lightbulb than the hardware store.