We should all remember that some great scientific discoveries _____________ by chance.
| A．gained | B．discovered | C．occurred | D．earned |
.The committee ______ over the problem among themselves for two hours.
| A．has argued | B．has been arguing | C．have argued | D．have been arguing |
.The public ______ generous in their contributions to the earthquake victims.
| A．is | B．was | C．are | D．has been |
.Cattle ______ on the hillside.
| A．grazes | B．is grazing | C．was grazing | D．were grazing |
.Her politics ______ neither conservative nor liberal.
| A．is | B．are | C．was | D．has been |
.Measles ______ a kind of infectious illness.
| A．is | B．are | C．were | D．have been |