1.
Choose the answer which best completes the sentence in suitable and accurate English.
'They ... ... asbestos into so many schools and hospitals, if they'd known back then how dangerous it ... ... .'
... shouldn't build ... / ... was ...
... would never have built ... / ... is ...
... would never have built ... / ... was ...
... shouldn't have built ... / ... was ...
' ... is' (Answer 2) is slightly better than '... was' (Answer 3), because asbestos remains dangerous even nowadays, and even if it's all removed and ceases to exist, the substance still retains that theoretical property. So the present tense remains 'universally true'.
Any Answer including 'shouldn't have' will carry more of a moral force: 'they wouldn't have ...' suggests that the builders can't be faulted for not knowing what they couldn't then have known (what Donald Rumsfeld would have called 'unknown unknowns'); but 'shouldn't have' suggests that they ought not even to have started, whether or not they were sure.