Which is correct?
In a _______ circuit, external noise can be picked up from appliances in the vicinity, from electrical transformers, from the atmosphere and even from outer space.
a. hard wired
The general rule for compound modifiers is that when they appear before the word they modify, they should be hyphenated.
A hyphen can make a big difference to the meaning of your sentence. For example, inviting 20-odd people to a party can make for a much different evening than inviting 20 odd people.
See more in the thousands of hyphens disappearing from the OED.
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