Which of the following contains a vague pronoun reference?

 - As David was walking with his friend Bob, he tripped on the uneven sidewalk.
 - As David was walking with his friend Bob, Bob tripped on the uneven sidewalk.
The first answer choice contains a vague pronoun reference. The pronoun he could refer to David or Bob.
The second answer choice shows a possible correction for the vague pronoun reference. He has been replaced with Bob.
As David was walking with his friend Bob, Bob tripped on the uneven sidewalk.