From the passage you posted, the sentence expressing certainty is 'This dark assertion will be illuminated later on'. We use 'will' when we believe that a future state is certain to occur. For the second sentence with an auxiliary verb, it is '<span>But if you'll unwrap that package you may see why you had me going a while at first'. However, it states a possibility, not a certainty.</span>
Seems that you have missed the letter so I attached it below. Here is my answer.
Based on the given letter, I can say that the best supporting statement that we can add to this letter is that most drivers in America have the age of eighteen and above. The answer for this would be option D. Hope this helps.
Use context clues to determine the meaning of the unknown idiom
Solution 1: The miller is employed at a mill and is worried that his anti-milling family & friend(s) might disapprove with his career choice(s).
Solution 2: The anti-miller is boycotting mills with his anti-milling family, friend(s) and/or coworker(s). This demonstrates his obvious lack of care for pro-miller’s opinion(s) of him while showing the opposite to the anti-millers. Consequently, his strong opposition towards mills won’t allow him to work at one and he’s now unemployed in a coincidentally mill-dominant city.
Solution 3: The miller is employed at a mill and is therefore highly regarded by his pro-milling family/friend(s)/coworkers.
Solution 4: The closeted anti-miller has no choice but to work at a mill due to his lack of self respect and untimely need for funds. The anti-miller’s choice(s) have begun to bring him a great deal of negativity from those whom know of his hidden anti-mill views. He’s unconfident in himself now that he’s essentially living a lie. His coworkers unknowingly, and rightfully so, add to his turmoil with their pro-mill commentary.
Solution 5: Everything and anything having to do with mills and milling is wiped from existence in it’s entirety. (This solution leaves you with no mill-related problem to have ever occurred and doesn’t give chance to any mill-related problem to happen.)