02br 00There is no absolute rule, but most are, as you say, in the singular (01i 00a shoe store, a five-minute break02i 00), while some are plural (01i 00systems engineer, cattle ranch02i 00). In your examples, 01i 00sales assistant02i 00 perhaps because the assistant's job is01i 00 sales02i 00;01i 00 sales02i 00 is his job-- it is a department name. 0-
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