Scientists have advanced many theories about why human beings cry tears,none of ______ has been proved.( )
whom
which
what
that
Having taken our seats, _______.
The students listened carefully to the teacher with their eyes _____ on the blackboard.
With our knowledge practice , we can make contribution to our society.
The most popular food ______ on menus in western countries is hamburger.
There is no greater pleasure than running on the playground, ____ after a ball.