Why did the Ming dynasty stop sending Zheng He on voyages and limiting outside influences?

What was the main point of helen hunt jackson's book, century of dishonor? a.it showed how unfairly american settlers were treated by the us government. b.it proved that american government was not responsive to the will of the people. c.it made the point that free land was no longer available to new settlers out west. d.it outlined the poor treatment of the native americans by the government.