The only sentence which is not a proposition is Sit down, because this is imperative, you are ordering somebody to do something, whereas propositions merely inform.
The correct answer is: adverb.
It's also referred to as adverb infinitive. It modifies "difficult", which is a predicate adjective (it gives more information about the subject of the sentence and there is a linking verb).
Answer:
I believe that opinions can be useful because if you have the correct idea and you use the right words, you can persuade someone in the correct manor.
Explanation:
I did 1 sentence, now lets see what u come up with.
the answer is a simple complement
It is a convex lens. It is used to produced to see a magnified version of text i.e.
Hope this helps! :D