047 第141题 请教

141. View the Exhibit and examine the descriptions for ORDERS and ORDER_ITEMS tables.
 Evaluate the following SQL statement:
 SELECT o.customer_id, oi.product_id, SUM(oi.unit_price*oi.quantity) "Order Amount"
 FROM order_items oi JOIN orders o
 ON oi.order_id = o.order_id
 GROUP BY CUBE (o.customer_id, oi.product_id);
 Which three statements are true regarding the output of this SQL statement? (Choose three.)
 A. It would return the subtotals for the Order Amount of every CUSTOMER_ID.
 B. It would return the subtotals for the Order Amount for every PRODUCT_ID.
 C. It would return the subtotals for the Order Amount of every PRODUCT_ID and CUSTOMER_ID as one
 group.
 D. It would return the subtotals for the Order Amount of every CUSTOMER_ID and PRODUCT_ID as one
 group.
 E. It would return only the grand total for the Order Amount of every CUSTOMER_ID and PRODUCT_ID
 as one group.
 Answer: ABD
 
c和d感觉是一样的啊，还是我理解有误。。。

排序後結果集是一樣的，但D的順序與cube中一致。期待專家解釋。