A - Because they are a direct link to the events in history
Primary sources are sources that came from that time of history from the same location, time period, and era. For instance, journals from World War II from a French Military leader. These are direct links to that time in history and are the most accurate sources that are presented.
B is incorrect as even without these sources there are other methods to know about history such as secondary sources.
C is incorrect as primary sources are not the only way - again, secondary sources are a thing.
D is incorrect as secondary sources are reliable and can be trusted, but they just aren't as important and well, direct as primary sources.
Hope this helps!
