Which is the best way to test the idea that lack of sleep causes attention problems?

A$.$Assign individuals randomly to different sleep conditions, then measure their performance on an attention$-$demanding task.

B$.$Ask people that report having attention$-$problems how well they sleep

C$.$Ask patients of an insomnia clinic if they have attention problems

D$.$Ask a number of random individuals how much they slept last night and also measure their performance on an attention$-$demanding task