Mr. Smith gave his wife money every Friday evening. His wife always spent it all the next Wednesday, so that for the next three days she had no money to spend at all. One day Mr. Smith asked her, “But how did you spend all that money?” She answered, “I don’t know.” So one Friday evening, Mr. Smith gave her money together with an exercise book and a pencil. He said to his wife, “Please write down how you spend the money.” Next Tuesday, his wife had no money again. Mr. Smith took the exercise book to see how she had spent it. “I have done what you told me,” she said gladly. She had written “Friday, I got 18 pounds from Mr. Smith.” on the first page, and on the second page, she wrote, “Tuesday, I have spent it all.”

如图所示，小东用力推放在水平地面上的木箱，木箱静止不动，下列说法正确的是（　　）

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