The English sentence could be either present of past tense. Therefore "Vitabu ambavyo walisoma vina picha nzuri" should also be accepted. Reported March '19
that's how I read it
no pun intended
I agree & that's exactly how I interpreted it myself. I'm beginning to think we the students are smarter than some of the teachers/moderators working for Duolingo!
Is it correct to say wanavyovisoma instead of ambavyo wanasoma?
Yes, I reported it today Aug 21 2019
Would it be correct to say?:
Vitabu walivisoma vina picha nzuri.
no, because '-li-' is the past tense infix. Also, the 'ambavyo' translates to 'which/that...'; another way to say it is 'Vitabu wanavyosoma vina picha nzuri'
past tense of that is 'Vitabu walivyosoma vina picha nzuri'
I said "walivyosoma" and it was marked wrong