Hidden Test and bits

Question

How to open hidden test with bits?
Do I need to become pro user to unlock hidden test?

And what is the use of bits?
why do I need that?

Answer 1

Bit's a non-pro (free) users feature. Non-pro version users can use those bits to unlock and try "pro content".

edit:

hidden test are not unlockable for any one.
https://www.sololearn.com/discuss/3093543/?ref=app