Which of the following contains a vague pronoun reference?
A. They have discovered that sleep has more than just restorative functions. For example, it plays a significant role in strengthening and solidifying memories.
B. Neuroscientists have discovered that sleep has more than just restorative functions. For example, it plays a significant role in strengthening and solidifying memories.
From the given choices, select the correct answer and reply with the letter of the chosen option.

A