Given a disfluent sentence, modify the sentence to it to its equivalent fluent form, preserving the meaning of the sentence.
One example: Who did the Han Chinese want to help the Khitan no I mean the Mongols fight?
Solution is here: Who did the Han Chinese want to help the Mongols fight?
Explanation: The input initially means Khitan and then changes to Mongols, hence this is correct.

Now, solve this: Which or rather what methods do petrologists use to examine rocks out in the field?
Solution:
What methods do petrologists use to examine rocks out in the field?