were electric slippers invented? —Sorry. I don’t know. [ ]

Look, the set of keys _ _ on the teacher’s desk.

The earthquake in Japan, measuring 9.0 on the Richter Scale, was so violent cause a series of explosions of the nuclear reactors in several power stations.

What are you going to do this weekend? —I _____ yet.

He goes to work early on workdays ______ Saturday and Sunday. [ ]