Scientists have advanced many theories about why human beings cry tears,none of ______ has been proved.( )
whom
which
what
that
________ the present unemployment continue, the society would face a more troublesome situation.
I forget where I learned the saying, or I _____ it to you now.
—Do you think George has passed the driving test?—No. If so, he ______ his car to our college yesterday.
— Did you see it with your own eyes, Mike? — Yes. Otherwise I _______you about it.
I didn’t know his telephone number, otherwise I him.