Can someone please help me with this question?:

Choose the progressive perfect form of the verb!

Antonio ________ along the river when someone began to pelt him with oranges.

Choose 1 answer:

(Choice A)
A
was walking

(Choice B)
B
had been walking

(Choice C)
C
had walked

Please don't give me a wrong answer. I'm trying so hard to get every one of these right