You will be given a definition of a task first, then an example. Follow the example to solve a new instance of the task.
In this task, you're given a sentence and question. Based on the information provided in a given sentence, you should identify the shortest continuous text span from the sentence that serves as an answer to the given question. Answer the question using coreference resolution. Coreference resolution is the task of clustering mentions in text that refer to the same underlying real world entities. For example let's take a sentence 'I voted for Obama because he was most aligned with my values, she said.' Here in this example 'I', 'my', and 'she' belong to the same cluster and 'Obama' and 'he' belong to the same cluster. Now let's discuss another example , original sentence: 'I voted for Trump because he was most aligned with my values',John said. Now here is the same sentence with resolved coreferences: 'John voted for Trump because Trump was most aligned with John's values',John said. 

Sentence: Sam pulled up a chair to the piano, but it was broken, so he had to sing instead. Question: What was broken?
Solution: piano
Why? The given sentence clearly states that Sam pulled up a chair to the piano, but piano was broken, so he had to sing instead. Therefore, the answer is piano. This is a positive example as the answer is correct and complete.

New input: Sentence: John promised Bill to leave, so an hour later he left. Question: Who left?
Solution:
John