Select the inverted verb that introduces the past hypothetical condition:
_____ the engineer not intervened promptly, the pressure valve would have failed under excessive thermal stress.
Answer and explanation
Correct answer: D. Had
- Step 1: Analyze the conditional construction: the main clause contains 'would have failed' (third conditional outcome). The 'if'-clause refers to a past unfulfilled counterfactual ('If the engineer had not intervened...').
- Step 2: Invert the third conditional by fronting 'Had': 'Had the engineer not intervened promptly...'.
Answer
Had.
Why not the others: 'Were' heads second conditional inversion; 'Should' heads first conditional inversion; 'Did' cannot form conditional inversion.
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