Answer:
The two things being compared are:
Diary and light
Explanation:
In this sentence, the "diary" is compared to a "light." The comparison shows that the diary provides a means of explaining some misunderstood facts. So, by discovering the diary, some information that has been missing about the issue will become available. This light will end the darkness created by the absence of adequate information. The diary points the reader to accurate, complete, and elucidating information.
