查看PRODUCT_INFORMATION表的描述: Name NULL? Type ------------------------------- PROD_ID NOT NULL NUMBER(2) PROD_NANE VARCRAR2 (10) LIST_PRICE NUMBER(6,2) 哪个查询可以正确得出LIST_PRICE列为空的个数？ A SELECT COUNT(list_price) FROM proceduct_information where list_price is NULL; B SELECT COUNT(list_price) FROM pruduct_information WHERE list_price=NULL; C SELECT COUNT(DISTINCT list_price) FROM produet_infomation WHERE list_price is NULL; D SELECT COUNT(nvl( list_price,0)) FROM pruduct_information WHERE list_price is NULL; D