Составьте вопросы к предложению. space exploration has been one of the most essential problems of our time.

Space exploration has been one of the most essential problems of our time. 
Has space exploration been one of the most essential problems of our time?Space exploration has been one of the most essential problems of our time,hasn`t it?
What kind of exploration has been one of the most essential problems of our time?
What has been one of the most essential problems of our time?
When has space exploration been one of the most essential problems?