Which of the following contains a vague pronoun reference?

 - Rick waved goodbye to Dan as his train pulled out of the station.
 - As Dan's train pulled out of the station, Rick waved goodbye to him.
The first answer choice contains a vague pronoun reference. The pronoun his could refer to Rick or Dan.
Rick waved goodbye to Dan as his train pulled out of the station.
The second answer choice shows a possible correction for the vague pronoun reference. The text has been rewritten so that the meaning is clear.
As Dan's train pulled out of the station, Rick waved goodbye to him.