First question; what kind of professor is this? English? Linguistics? Economics?

Statement 1) looks like the kind that a rabid prescrip would gladly expend the effort required to misunderstand. The juncture of "multinational companies" and "are often called the economic approach" lends itself to this purposeful misunderstanding. Beyond that I leave it to the more attuned to this sort of thing to comment.

Regarding statement 2) I might suggest 'accessible' rather than 'achievable.'

Statement 3) is a little awkward but it would take some thought to fix it.

The "a" in statement 4) looks perfectly good to me but I might change "knowledge" to "understanding".