Once _____ , the machine will keep running.
| A．start | B．started | C．to start | D．starting |
33.A reward of 1,000 dollars will be given ________ can find the lost child.
| A．whomever | B．whoever that | C．to whomever | D．to whoever |
32.So often ______ in recent years that it has almost become a rule.
| A．has this happened | B．this has happened |
| C．is this happening | D．for this to happen |
31.The farmer who admitted ______ the tiger was severely punished.
| A．to kill | B．killing | C．killed | D．to have killed |
30.They ______ be at home. You see, the door is locked from the outside.
| A．can’t | B．may | C． must | D． mustn’t |
29.Our English teacher had us all _______our names on a card.
| A．written | B．to write | C．write | D．for writing |