Why does the united states see north korea as a threat to our national security? how is the government trying to resolve the situation? what monumental event just occurred between north and south korea?

As far as a threat, North Korea has been seen as such for many years (since WWII), when North Korea sided with Russia. America and its Capitalism were not fond of the Communist party, and so we have really been fighting/being concerned about Russian influence.

Historically North and South Korea have been in conflict many years before America and Russia got involved. When we did though it got worse.

Trump has done very little to resolve this, some would say he is making it worse.

AND as for any current events ask Google!

The correct answer is:   [b]:   "physical systems" .