You cannot put any sort of endmark (period, question mark, or exclamation point) here because it is not a complete sentence. Complete sentences must have a subject and a predicate. You can tell there's not even a verb here, so this is just a fragment, and we would have to use a comma.
a. Very truly yours,
He have four effects of the introduction of steam-powered machinery, and they were all things that did happen. So the answer will be looking for a non-economic effect rather than for something that was not an effect.
Such non-economic effect is the increase in pollution. It is an environmental effect that might have some very mild economic effects (it might for example decrease tourism), but it's not generally an economic effect.
