Questões de Concurso Militar EEAR 2012 para Sargento da Aeronáutica - Aeronavegantes e Não-Aeronavegantes

Since astronaut Yuri Gagarin became the first man to travel in space in 1961, scientists _________ what effects space travel has on the human body.

(Adapted from Active Skills)

Fill in the blank with the correct verb tense.