Dear friend, the zero article is often used with parallel structures like X and Y , where X and Y are identical or contrasting nouns: chenyincheng They are father and son. - correct ; The a y are a father and a son. - correct, but I see it as less preferable given the idiomaticness of the first construction chenyincheng They are cat and dog.
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chenyinchengThey are father and son. - correct;
Theay are a father and a son. - correct, but I see it as less preferable given the idiomaticness of the first construction