Sentence 1: Zack's son wanted a bike for his birthday. 
 Sentence 3: While Zack waited at a bus stop one day, he saw a bike with no lock 
 Sentence 4:  So he grabbed it and took in on the bus with him 
 Sentence 5:  When Zack's son got the bike for his birthday, he was ecstatic 
 Option 1: Zack's sister had promised to help him with the costs. 
 Option 2: But he knew he couldn't afford one.