The answer is D all of the above
He launched Operation Linebacker. He bombed Hanoi and Haiphong that led to the
destruction of the North’s economic and industrial centers. He continued the bombings and also threatened to withdraw U.S. and end the agreement. Nixon halted the bombings and signed the
Paris Peace Accord that ended U.S. involvement in the area.
Frontier--the land beyond the settled and established areas of the US was referred to as the frontier regardless of movement west.
The idea of the frontier and people willing to take the risk to settle it contributed to basic values of risk-taking, hard work, individualism, and progress essential to the creation of the US.