Continue heating until steam _____.
| A．has appeared | B．appear | C．appeared | D．will appear |
It was probably at that moment _____ I understood what kindness really was.
| A．that |
| B．when |
| C．which |
| D．what |
—What was it that caused the party to be put off?
—________ the invitations.
| A．Because Tom delayed sending |
| B．Tom delayed to send |
| C．That Tom delayed sending |
| D．Tom delayed sending |
Who ________ that played a trick on the new English teacher?
| A．it was |
| B．was it |
| C．he was |
| D．was him |
—Where did you pick up the stamp?
—It was at the post office ________ we worked in the summer vacation.
| A．that |
| B．where |
| C．which |
| D．when |
—Was it by cutting down staff ________ she saved the firm?
—No，it was by improving work efficiency.
| A．when |
| B．what |
| C．how |
| D．that |