Exercício: Have studied ou had studied?

Olá, pessoal

Fiquei com uma dúvida e gostaria de saber se alguém pode saná-la.

7 Choose the best alternative to fill in the gaps:

They ________ in that school for a long time when they decided ________ .

( )had studied / to move
( )have studied / move
( )studied / to move
( )had been study / to move

Qual seria a alternativa correta e por que?

Obrigado!