Т.е так просто? Без всяких had been studying ?😁

http://forum.wordreference.com/threads/i-have-studied-i-studied-i-have-been-studying.930731/

"If you've finished, you can use either "I have studied" or "I studied" "

However, this doesn't dismiss "had been studying" option. https://www.google.co.uk/search?q=had+been+studying shows very few results for "had been". I *think* it's used when the date is important when one finished studying or graduated.

Oksana, "had been doing" это Past Perfect Continuous, это время употребляется всего в двух случаях:
1. Показать длительность действия в прошлом, которое происходило ДО другого действия, произошедшего в прошлом (Kate had been waiting for her sister for more than an hour before she arrived).
2. Чтобы показать причину чего-либо, что произошло в прошлом (She passed the test because she had been studying very hard).

grumbler, "had been studying" can't be used here.