Zip codes have been used as a way for post offices for the last 50 years to send mail efficiently. the five digits allow the post office to locate where a person or a business wants to send a letter or a parcel. the first three numbers indicate the sectional center facility or major post office in the area, and the last two numbers signify the delivery area. in the 1980s, the post office introduced the addition of four more numbers. this narrows the area down to a block or a building. to make mailing more efficient, unique zip codes could be given to places that receive and send a lot of mail. these places can be a company or a university. because of its various uses, zip codes are beneficial in distribute mail more easily.

based on the passage, what is the meaning of unique zip codes?
a.
codes that note the sectional center facility and area
b.
codes assigned to entities with sufficient mail volume
c.
codes given to places with certain amount of employees
d.
codes that identify each and every u. s. company