Which form completes “What would our lives be if X-ray machines ...”?

Question

What would our lives be if X-ray machines ...

Answer choices

1. were not invented
2. would not have been invented
3. had not invented
4. had not been invented

Answer

had not been invented — option 4.

Explanation

The condition concerns a past event: the invention of X-ray machines. An unreal past condition uses the Past Perfect. Since the machines receive the action of invention, the passive form is required:

had not + been + invented.

The completed sentence is What would our lives be if X-ray machines had not been invented? The result describes a hypothetical state of our lives, while the condition imagines a different past.