Describing an action that happened before another one in the past

You were asked to fill in the blank. You chose the correct answer.

Don't overthink.

Dfda Punk is right of course. Technically 'were getting' might apply to a situation where they arrived while you were having some difficulty reaching the facility. However I still wouldn't use it unless context absolutely somehow required it

You seem to be a newcomer and a few of us curious and caring folk might like to know something about you

You should be warned incidentally about the TAATANE who will attack no matter how innocent

https://able2know.org/topic/369658-1

You write:
“Isn't it true that the action that happens first takes past perfect and the action that follows takes simple past?”

You mean: “ He had been there when I arrived.” ????
When the intended meaning was “He was there when I arrived.”

Remember, too, that we are talking about the sequence of actions in a sentence, so:
Is it:
“Before I joined Microsoft, I was employed by Oracle for ten years.”
Or
“Before I had joined Microsoft, I was employed by Oracle for ten years.” Here, Past Perfect places the action of taking the second job as occurring prior to the first job at Oracle!

When you write, “Isn't it true that the action that happens first takes past perfect and the action that follows takes simple past?” , you are referring to the case where Past Perfect is used to signal a change, or separation from preceding events. Some examples:

1. Paul was an alcoholic and unable to hold down a job. He turned to crime to obtain money to buy alcohol, and so became involved with a large section of the criminal element in the community. He came into our rehabilitation program, and gradually stopped drinking altogether. After he had given up drinking, he was then able to shed his old criminal cronies, obtain employment, and become a useful and productive member of society.

‘after + Past Perfect’ signals (and emphasizes) a separation between ‘before’ and what then happened – it contrasts his life before, as an alcoholic, and then, how he turned his life completely around.
Subsequent events are then in Simple Past.

2. It was nearly 5 p.m. when by boss informed me I was to give a presentation at one of our branches in another city. No worries, I thought – I have all evening to prepare it. When I arrived home, I was plunged into a domestic ((events in simple Past))…I managed about three hours of rest, then skipped breakfast and dashed to the station. I arrived just as the train drew in, and the moment the carriage door opened, I boarded the train. As soon as I had found my seat, I opened my laptop and began to write my presentation.

There is an abrupt use of the Past Perfect, the implication being, whew, I’ve made it – I’ve caught the train on time…..now, NOW.. I can concentrate on preparing my report. All that went before is now separated off from what I really need to get on and do - prepare my presentation.