The answer would be the 3rd sentence.
<span>Gideon and Francis took a difficult - but not interesting - science course.</span>
Your tone
Let's look at the other choices first. Your credibility is your ability to be believed as trustworthy in regards to the subject. Whether you're talking to a group of teenagers or professional adults, your credibility doesn't change. Your clarity also should not change based on the audience because you want to be clear in your message at all times. Also, your purpose should be the same no matter the audience. The only thing that would change is your tone. The way you approach a crowd of teenagers would vary greatly from the way you'd approach a room of professionals.
I would say they should once they are 18 because their brain should be developed enough to know right from wrong. Unless they have a mental disability.
Childhood, green is the third room and childhood is the third answer.
