It is important to keep ______ in an emergency (紧急情况).
| A．quiet | B．calm | C．silent | D．still |
—You can't imagine ________ fantastic fun online chatting is.
—Really？But it may cause a lot of trouble sometimes.
| A．what | B．how | C．why | D．so |
It is pretty well understood ________ controls the flow of carbon dioxide in and out the atmosphere today.
| A．that | B．when | C．what | D．how |
A plan has been put forward ________ more graduates should go to work in the country.
| A．when | B．that | C．whether | D．how |
After three hours'climbing，they reached ________ they thought was the place they'd been dreaming of.
| A．what | B．which | C．where | D．that |
Engines are to machines ________ hearts are to animals.
| A．as | B．that | C．what | D．which |