Question

I____(be) at the scene of the crime 15 minutes ago.​

Answer 1

Resposta: I was at the crime scene 15 minutes ago.

Explicação: Se utiliza "was" para pronomes como "I" "he" "she" "it" se não se utiliza o verbo to be "were" como por exemplo "you were" "they were" "we were". Was e Were são as formas do verbo to be no passado.

Answer 2

Resposta:

I was at the scene of the crime 15 minutes ago.​

É was porque diz que foi 15 minutes atras, e pq o verbo então se conjuga com escrito abaixo,

I was

you were

he was

she was

it was

we were

you were

they were