Which event most contributed to the changing troop levels shown in this graph?
the twenty-sixth amendment lowered the draft age to 18 from 21.
u. s. and north vietnamese ships exchanged fire in the gulf of tonkin.
congress expanded presidential powers to wage war under the war powers act.
communist troops launched a series of attacks during the tet offensive.

U.S. and North Vietnamese ships exchanged fire in the Gulf of Tonkin.

The answer is:

U.S. and North Vietnamese ships exchanged fire in the Gulf of Tonkin.

The Gulf of Tonkin was used by different American ships to spy on North Vietnamese radar towers, on July the submarine encountered three Soviet-made Vietnamese submarines and opened fire, this started the war on Vietnam as several shots were fired and after that the president passed the Tonin Gulf Resolution that gave him the power to declare war in southasia as he saw fit, after this more and more troops were deployed in Vietnam.

the columbian exchange, also known as the columbian interchange, named for christopher columbus, was the widespread transfer of plants, animals, culture, human populations, technology, and ideas between the americas, west africa, and the old world in the 15th and 16th centuries, related to european colonization and trade following christopher columbus's 1492 voyage. invasive species, including communicable diseases, were a byproduct of the exchange. the changes in agriculture significantly altered and changed global populations. the most significant immediate impact of the columbian exchange was the cultural exchanges and the transfer of people between continents.

the new contact between the global population circulated a wide variety of crops and livestock, which supported increases in population in both hemispheres, although diseases initially caused precipitous declines in the numbers of indigenous peoples of the americas. traders returned to europe with maize, potatoes, and tomatoes, which became very important crops in europe by the 18th century.

the term was first used in 1972 by american historian alfred w. crosby in his environmental history book the columbian exchange. it was rapidly adopted by other historians and journalists and has become widely known.