Complex sentence: Arrigoitia continued her studies and, in 1966, became the first person in the University of Puerto Rico to earn a Masters Degree in history with her thesis "La Diputacion Provincial de Puerto Rico, 1820-23".
Split into simpler sentences: Arrigoitia continued her studies and, in 1966, became the first person in the University of Puerto Rico to earn a Masters Degree in history. Her thesis was "La Diputacion Provincial de Puerto Rico, 1820-23".