The correct answer is A. saw.
To explain why the correct answer is "saw," let's break down the sentence and examine the verb tense and context.
1. Sentence Structure:- The sentence begins with "I," indicating that the speaker is referring to themselves.
- The verb phrase is "an old classmate of mine at the library," which describes the action or state of the subject (the speaker).
2. Verb Tense:- The verb tense used in the sentence is past tense because the speaker is referring to a past event.
- Since the sentence does not provide any specific time frame, we can assume that the action took place at some point in the past.
3. Verb Choice:- The word "saw" is the appropriate verb choice in this context because it is the simple past tense of the verb "see."
- When we "see" someone, it means that we visually perceive their presence or encounter them with our eyes.
4. Other Options:- The option "was seeing" is incorrect because it uses the past continuous tense, which implies an ongoing action in the past. However, the sentence does not suggest ongoing action but rather a single event of seeing the classmate at the library.
- The option "none of the above" is also incorrect because the correct answer is "saw."
Therefore, the correct answer is A. saw.