It meant that as soon as one country falls under communist control, another country would fall and so on. Vietnam at the time was split into North (communist) and South (being attacked by North) and the US didn't want communism to spread, reasoning that it might eventually come to the US if nothing was done.
Let them through the more the merrier.
the headright system is the system that promised land to people who agreed to settle in Virginia.
C is the answer, sorry if I’m wrong
