mutant table?

Hi guys, sorry, but I have absolutely no idea how to even approach this problem, or where to even start. I don't even know what you call this table!

Complete the table. Several auxotrophic point mutants in Neurospora are treated with various agents to see if reversion will take place. The following results were obtained (a plus sign indicates reversion; HA causes only G • C → A • T transitions).

Mutan   5-BU   HA   Proflavin   Spontaneous reversion   a) The nature of the original mutation event   b) Name a possible mutagen (hint: this should be a group name, not a specific name such as 5-BU)
Sample   +    –    –    +   transition   Base analogs

1

   –    –    –    –      
2


    –    –    +    +      
3



    –    –    –    +      
4


   +    +    –   +      

a. For each of the five mutants, describe the nature of the original mutation event (hint: your answer should not be the reversion) at the molecular level.

b. For each of the five mutants, name a possible mutagen that could have caused the original mutation event. (Spontaneous mutation is not an acceptable answer.)

ehhhh sorry about the table formatting getting all mixed up :/