Identify the correct verb form to complete the following sentence: "The team ___...

Questão de Inglês da banca Avança SP aplicada no concurso Prefeitura de Ubatuba SP (2024). Confira a resolução completa abaixo:

Identify the correct verb form to complete the following sentence:
"The team ________ working on the project for several months when they finally reached a breakthrough."