Past simple or: Why ... the navigators ... to find new trade routes in the 15th.

[Penuel]

Hello, please help me to choose the right answer.

Why ... the navigators ... to find new trade routes in the 15th century?
a) do ... tried
b) has ... tried
c) did tried
d) had ... tried

If any of these answers is correct please let me know why.

Thanks.

[Kitosdad]

d) Is the correct form. The sentence, and the answers, are all couched in the past tense.

Kitosdad.
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