Answer :
i think the correct subordination conjunction to complete the sentence would be : D. whatever
Whatever he did to study for the final exam must be have been helpful; his grade improved from a B to an A. The other one seems not to make any sense
hope this helps
Whatever he did to study for the final exam must be have been helpful; his grade improved from a B to an A. The other one seems not to make any sense
hope this helps
The actions that the person in the sentence did remain a mystery. When all the information is not known what we usually do is make a deduction. The relative pronoun necessary in this specific example is "Whatever". The importance of the action is not relevant but just a means to achieve the final result, which was a really positive one.