---I’m sorry, but there is no smoking on this flight.
---Oh, I _____that. Sorry, I won’t again.
| A．don’t know | B．didn’t know | C．won’t know | D．haven’t known |
This is one of the means ____t he electic energy is conducted from one place to another.
| A．by which | B．by that | C．through which | D．through that |
The buses, ____were already full, were surrounded by an angry crowd.
| A．most of them | B．most of which | C．which most | D．that most |
Can you solve such problems ___raised by the audience?
| A．what were | B．as were | C．that were | D．which were |
We often think of the days ___we spent together on the island.
| A．when | B．which | C．in which | D．during which |
Air, _ __we breathe every day, is around us all the time.
| A．that | B．/ | C．which | D．it |