Could you guys help me answer these questions? Thanks a lot.

In the "Word Count" example, what is used as a "key" for the "reduce" operation? The first letter of each word The first word of each sentence The whole sentence itself Individual word Suppose you're processing a large text file line by line. Which is not a valid map operation? Dropping lines that does not end with period (.) Finding sentences/lines that have at least one word in common Filtering lines based on the presence of a certain word Truncating lines by deleting small but frequent stop words, such as "a,", "of", "and", "the", etc