Answer:
The answers are options C and D.
Step-by-step explanation:
If the intention is to improve reader's ability to understand the military leaders' fear that the Navajo code would no longer be effective it is necessary to know if anny Japanese or German student learn the language; if they did so, it wouldn't have been possible to use it again; also, it would be logical to ask what kind of code was used by the American Indians during World War I; this way, the readers would know if enough precautions were taken by the military leaders about the new codes.