it would be possible.改否定（内详）

it would be possible.改否定（内详）
It would be possible to have an observatory in our shcool.（改为否定句）
_____ _____ _____ _____to have an observatory in our shcool.
可不可以改成It wouldn't be possible to have an observatory in our shcool.
把possible 改成impossible是答案，那么我这个改句算错么？would改成wouldn't有问题么？