Sentence 1: Mark said if Nancy mowed the lawn he would buy her a root beer float. 
 Sentence 3: When she finished Mark went to go get her a root beer float 
 Sentence 4:  When Mark returned he only had a empty cup 
 Sentence 5:  Mark said he got thirsty and drank the root beer float 
 Option 1: Nancy did not mow the lawn though. 
 Option 2: Nancy mowed the lawn.