What can cause an open item on the goods receipt/invoice receipt (GR/IR) account?

What can cause an open item on the goods receipt/invoice receipt (GR/IR) account?
A . Difference between quantity ordered and quantity invoiced for a purchase order item.
B . Difference between quantity delivered and quantity invoiced for a purchase order item
C . Difference between purchase order price and invoice price for a purchase order item
D . Difference between material valuation price and invoice price for a purchase order item

Answer: B