What made the Twin Towers come down? After the Twin Towers had collapsed, engineers tried to find out what could have made a difference.

Gap-fill exercise

Complete the if-clauses: might/would/could (not) + have + past participle.
support the buildings better
If the steel columns had been stronger, they .

be less heat
If there hadn't been so much jet fuel, there .

floors not crash down
If there had been less heat, .

more people escape
If the stairways hadn't all been in the centre of the buildings, .

cannot crash through them
If the walls had been thicker, the planes through them.