单选题：根据下列材料请回答题： Once a man and his wife worked for a businessman

根据下列材料请回答题： Once a man and his wife worked for a businessman．There was a big box in the businessman’Sliving room．The businessman pointed at the box and said，“There’s only one thing you 16 do．Don’t open the box．”Mter saying this，he 17 the living room．
The woman said to her husband，“There 18 be something expensive in the box．Let’s 19 it，shall we?”Her husband said no to her．
The woman didn’t give up her 20 ．One day，she decided to find out what was 21 it．Her husband didn’t stop her．She opened the box and looked22She found nothing in the box and tried hard to close it， 23 she failed．
That evening the businessman came home and found the box was 24 ．He was very angry and asked the woman and her husband to leave his home．
“But there was nothing in the box，”the woman said．“We haven’t taken anything 25 "The businessman shouted at them：“The box is not important．I cannot believe you．1'hat’s important．” A．needn’t
B．wouldn’t
C．mustn’t
D．can