The bus ____ at six, but it arrived an hour later.
A．should arrive B．had arrived
C．should have arrived D．must arrive
The hotel ____ me ＄50 for a single room during one night.
A．took B．cost C．charged D．paid
Little ______ that we were watching his every move，so he seemed to be going his own way in this business.
A．did he know B．he didn't know C．didn't he know D．he knew
When the woman was told that her husband had a car accident, her mind _____.
A．ended up B．made sense C．blocked out D．went blank
Color-blind people often find it difficult to _____ between blue and green.
A．compare B．separate C．distinguish D．divide
Scientists say it may be five or six years ______ it is possible to test this medicine on human patients.
A．before B．after C． since D．when
