Translation:They buy not only computers but also telephones.
using 'mot' would indicate 'a' rather than 'the'. My question is really about why we should assume that computers and phones here are plural, when there is no plural indicator. Usually Duo always wants us to say the+singular unless we have the word 'mot' for one/a or nhung/cac for plurality. Here the nhung is part of a nhung...ma construction, not a plural indicator, so I want to know what indicates plurality here, and therefore why singular nouns with 'a' or 'the' are incorrect. Incidentally, in practice you don't need to say 'mot' to indicate that you mean 'a' instead of 'the', though Duo usually wants you to do that.