What letters should be here?

I'm having difficulty with another quiz.

By the way, thanks to everyone who answered my previous question or who wants to think over it :)

What letters should I put in place of the question mark?

            A

B    C

?    ?    ?

C    A    D    B

B    D    A    C    B

A    D    B    C    A    D

• Where are you getting these questions? It's generally frowned upon not to attribute puzzles to their source if they aren't original. – Austin Weaver Apr 1 '18 at 17:39
• From the Russian-language collection of tests. Therefore, I see no point in giving a link here. – Alex Petrov Apr 1 '18 at 17:47
• @AlexPetrov why are you using two accounts? this is generally discouraged. – Quintec Apr 1 '18 at 18:03
• Giving credit to the original author makes a big difference when distinguishing between fair use and copyright infringement. This is kind of an important thing, since this website belongs to an American company. We don't need to see the source, we need to see that it is properly mentioned. – Bass Apr 1 '18 at 18:05
• Why did you delete the bottom lines of the puzzle? – puzzledPig Apr 1 '18 at 18:58

from the fourth row we have C A, to obtain the question mark above them we look for other instance in the triangle that has C A and look above it. In this case, the last row. We can also verify that since C B gives a B, (example from the last two columns in the 5th row).