Mr. and Mrs. Green were going on their vacation with their children, and they had to arrive at the airport before 11:40 the next morning. "It takes us thirty minutes to get there in a car," Mr. Green said, "so we must get ready before eleven o'clock. Don't be late." At ten fifty, all of them were busy doing something, but Mrs. Green wasn't. She sat quietly in a chair in the garden and enjoyed the good weather. Mr. Green and the children were surprised, but Mrs. Green said, "Don't worry! Before I went to bed last night. I made all our clocks and watches twenty minutes ahead . Now we can get to the airport early." Why did the Greens have to be ready before eleven o'clock?
Because they were going to a party. Because they didn't go to the airport. Because they had to go to the airport by bus. Because they must arrive at the airport before 11:40.