I said "They can go by car or train." I was marked wrong for not saying "by car or by train." Why should I repeat "by" a second time when "bằng" appears only once in the Vietnamese sentence. After all, both phrasings are correct in English, and I suspect both would sound OK in Vietnamese, too. "bằng xe hơi hoặc bằng tàu hoả"?
Maybe the DL team forgot to add that as an acceptable translation. I agree with you, and I think you should report it.
yes, DUO, when using "and" or "or" the preposition does not need to be repeated (in English). It is not wrong to repeat it, but the usual format is to not. It is more unusual to say "by car or by train."
This goes for other prepositions as well. For instance, you would more likely say, "She is writing letters TO her aunt and her brother" than "She is writing letters to her aunt and to her brother." That's just how we roll.
This is so frustratingly trifling. After a year, I have no expectation this will change and just have to memorize more goofiness. Mnemonic for this: "They go by by."
And god forbid you use "the".