Our people are losing faith, not only in government itself but in their ability as citizens to serve as the ultimate rulers and shapers of our democracy. "We were sure that ours was a nation on the ballot, not the bullet, until the murders of John Kennedy and Robert Kennedy and Martin Luther King Jr. We were taught that our armies were always invincible and our causes were always just, only to suffer the agony of Vietnam. We respected the Presidency as a place of honor until the shock of Watergate. "We remember when the phrase ‘sound as a dollar’ was an expression of absolute dependability, until ten years of inflation began to shrink our dollar and our savings. We believed that our Nation’s resources were limitless until 1973, when we had to face a growing dependence on foreign oil. Jimmy Carter, Public Papers of the Presidents of the United States, 1979
Which of the following would most strongly support Carter's contention in his phrase "we were taught that our armies were always invincible"?

a. The secret expansion of the Vietnam War into Cambodia by Nixon.
b. The killing of protesters at Kent State and Jackson State Universities.
c. The decision by newspapers to publish the Pentagon Papers.
d. The defeat of South Vietnam by North Vietnam in 1975.