—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
The last time we had great fun was ________ we were visiting the Water Park.
A．where B．how C．when D．why
