A)By the time I arrived, they will have finished their project.
B)By the time I arrived, they would have finished their project.
C)By the time I arrived, they had finished their project.✓ Gabarito
D)By the time I arrived, they have finished their project.
E)By the time I arrived, they were finishing their project.
Explicação
A sentença correta é 'By the time I arrived, they had finished their project.' pois expressa uma ação passada que ocorreu antes de outra ação passada, utilizando o passado perfeito.
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