In "The Unbreakable Code," why did John’s grandfather speak sternly when talking about the Navajo language?

A. He had forgotten Navajo when he was younger as well.

B. He was afraid John’s friends would keep him from speaking Navajo.

C. His grandfather had taught him Navajo

D. The Navajo language had helped his country win WWII.

PLS HELP I WILL MARK BRAINLIST IF CORRECT!! :)