Scientists have advanced many theories about why human beings cry tears,none of ______ has been proved.( )
whom
which
what
that
Professor John will give us a lecture on how to learn English, but when and where ____ yet.
He asked me how much ____ on the mountain bike.
I haven’t finished my report, but I promise I ____ it by the end of the week.
—Haven’t seen you for ages! Where have you been?—I went to Ning Xia and ____ there for a year, teaching as a volunteer.
“It’s too early to conclude that the problem ____ but I think we have been moving in the right direction,” the manager said