a) The author asked Johnson the reason for him being so sad.
b) Johnson replied that he was sad because they killed one of Johnson's three friend.
Step-by-step explanation:
The above question includes a conversation between the author and Johnson.
The conversation goes like,
Author: Why are you so sad ?
Johnson: They have killed one of my three friends.
From the above conversation we can clearly understand that Johnson was sad because one of his three friend was killed and now with this understanding we can answer the following.
a)what did author asked to Johnson?
The author asked Johnson the reason for him being so sad.
b) what was Johnson's reply?
Johnson replied that he was sad because they killed one of Johnson's three friend.