Question

read the passage.
in the mid-nineteenth century, milk needed to be consumed almost immediately because people did not have an effective method for keeping it fresh. most people assumed that turning sour was simply what milk did, but a researcher named louis pasteur had a different theory. pasteur suspected that the air is filled with tiny microorganisms. he believed that when these microorganisms enter our food and drinks, they cause chemical changes to occur. pasteur found that when substances like milk are heated to a high temperature and sealed in an airtight container, it dramatically increases how long they remain fresh. a version of this process, which came to be known as pasteurization, is still being used by dairy products today.
based on clues in the passage, what is the most likely reason why pasteur heated the milk to a high temperature?
to kill off microorganisms already present in the milk
to make it more appealing to people who liked warm milk
to prevent new microorganisms from entering the milk
to generate microorganisms that prevent milk from spoiling

Explanation:

The passage states Pasteur believed tiny microorganisms in air cause food/drink spoilage, and heating substances like milk to high temperatures then sealing them drastically extends freshness. This means heating targets the microorganisms that spoil milk.

Answer:

to kill off microorganisms already present in the milk