Cuándo se desconoce si la persona es un él o un ella, generalmente utiliza their.

Don't worry, if you were saying that in English you would be completely understood. To me as an English speaker they should both be accepted. I was doing timed practice and entered "their advertising department has been slow" and it would not accept that either. My other problem was that my time ran out before I could report that my answer should be accepted.

cuando no se sabe si el sujeto es masculino o femenino muchas veces se usa they/them/their para reemplazar his/her pero sigue siendo singular. espero haber respondido tu pregunta!