Question

My new co-workers (had been, were) very friendly to me until they learned my salary was considerably higher than theirs.

Short answer

The correct sentence is: 'My new co-workers were very friendly to me until they learned my salary was considerably higher than theirs.'

Step-by-step solution

Identify the Time Indicators in the Sentence

The sentence does not provide explicit time indicators, such as dates or specific moments. The only time-related information given is 'until', showing there is a change in the situation at some point.

Analyze the Sequence of Events

The co-workers' friendliness shifted upon the revelation about the salary. Therefore, first, they were friendly, and then they learned about the salary and changed their behavior.

Select the Correct Tense

Considering the sequence of events and as the change happened at a specific moment in the past, the past simple 'were' is the correct verb form to use in this context.