Thanks for the sentence pointing out how similar March and September sound :)
For a Spanish speaker, this is tricky. In Mexican Spanish, B and V are almost the same sound, as are C, S, and Z. I got it wrong on my first try because I automatically thought березень and вересень were the same word :P
Wrong because i missed a soft sign? Really?
To a native speaker it is quite a significant sound difference, maybe like the difference between pig and big