Which of the following contains a vague pronoun reference?

 - Jake went on to calculus after studying trigonometry, but he never fully comprehended calculus.
 - Jake went on to calculus after studying trigonometry, but he never fully comprehended it.
The first answer choice contains a vague pronoun reference. The pronoun it could refer to calculus or trigonometry.
The second answer choice shows a possible correction for the vague pronoun reference. It has been replaced with calculus.
Jake went on to calculus after studying trigonometry, but he never fully comprehended calculus.