________carefully if any change occurs when doing experiments in the lab.
| A．Observe | B．To observe |
| C．Observed | D．Observing |
The police ______ for the lost child everywhere.
| A．is looking | B．has looked | C．are looking | D．are looked |
A group of children ______ hide and seek in the garden.
| A．is playing | B．are playing | C．has been playing | D．was playing |
Two kilograms ______ too heavy for a sick man.
| A．is | B．are | C．was | D．were |
Many a school and many a hospital ______ since 1949.
| A．have set up | B．has set up | C．have been set up | D．has been set up |
All that can be said ______.
| A．has been said | B．have been said | C．is said | D．are to be said |