What is the perfect condition for sleep?

• A、 A quiet room with flashing lights.

• B、 A bright room with white curtains.

• C、 A cool room with comfortable pillows.

• D、 A dark room with electronic devices.

Since computers were introduced, they____ to be useful tools for people weak in math.

• A、had proved

• B、have proved

• C、 will prove

• D、are proving

56.（）