The correct answer is the following.
The participants in the protest march felst very disappointedly and angered for the U.S. participation in the Vietnam War. Sadness can be another word for their frustration.
On October 21, 1967, almost 100,000 people gathered in Washington D.C, to participate in the march to protest against the Vietnam War. Half of those people marched to the Pentagon to express their anger and frustration for the United States involvement in the war. This protest was a clear demonstration for the opposition of President Lyndon B. Jhonson war effort.