When do backorders occur?

Question $9$Select one:

a$.$

When you don't have sufficient inventory to fill an order

b$.$

When the customer decides to keep their business with your organization.

c$.$

When the customer has enough inventory all ready and cancels the order.

d$.$

When your customer decides to take her future business elsewhere.

e$.$

When you don't have sufficient inventory to fill an order and when the customer decides to keep their business with your organization.