---- the spacecraft's main thrusters failed, redundant auxiliary engines would automatically engage within milliseconds.
Answer and explanation
Correct answer: D. Should
- Step 1: This is an inverted Type 1 conditional sentence replacing 'If the thrusters should fail': 'Should + subject + bare verb'.
- Step 2: 'Should the spacecraft's main thrusters fail, auxiliary engines would engage'.
Answer
Should.
Why not the others: 'Were' requires an infinitive complement ('Were the thrusters to fail'); 'Had' requires a past participle ('Had failed'); 'Could' is not a conditional inversion particle.
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