Sentence Correction

hello,

i'm not sure why it's was earning .. hm .. my 2nd attempt was C .. there was xxx and it earned .. so then the sentence will be consistent with the past simple tense .. but the correct ans should be A ..

could someone explain why ?

THanks.

This is actually an example of past progressive, which shows something that was happening when another thing occured. The past perfect shows order of things in the past: one thing happened and then another thing happened.

Ex. "The student was studying when the fire alarm rang." "They were studying when the fire alarm broke out."