How do you know whether a detail is implicit?

if you are describing something that is clearly explained or expressed, the word to use is Explicit; if you are describing something that is implied, Implicit is the word.

So basically, if you aren't be specific about what it is you're talking about, it would be Implicit. If it were Explicit, you'd be specific about whatever it is you're doing or talking about.

you do this because a real teammate puts the team first

answer is b,

"the tracer looks harmless in flight, but the pilot’s bitten lips and white knuckles suggest how dangerous it can be."

it was correct.