14、（6-13） choose the best answer:
View the Exhibit and examine the structure of the ORDERS table.
Which UPDATE statement is valid?

A) UPDATE orders
SET order_date = '12-mar-2007'
AND order_total = TO_NUMBER(NULL)
WHERE order_id = 2455;

B) UPDATE orders
SET order_date = '12-mar-2007',
order_total = NULL
WHERE order_id = 2455;

C) UPDATE orders
SET order_date = TO DATE('12-mar-2007','dd-mon-yyyy'),
SET order_total = TO NUMBER(NULL)
WHERE order_id = 2455;

D) UPDATE orders
SET order_date = '12-mar-2007',
order_total IS NULL
WHERE order_id = 2455;

Answer：D
(解析：注意语法)