Every possible means _______to prevent the air pollution, but the sky is still not clear.
| A．are being used | B．have been used | C．has been used | D．had been used |
Not until she took off her dark glasses ________ she was a famous film star.
| A．I realised | B．that did I realise | C．did I realise | D．that I realised |
It is ten years since we ________ each other last time.
| A．see | B．have seen | C．saw | D．had seen |
More wild tigers have been seen in the forest around this area, ________ there used to be very few.
| A．when | B．where | C．what | D．which |
It is usually warm in my hometown in March, but it ________ be rather cold sometimes.
| A．must | B．can | C．should | D．would |
You can never be ________ when crossing the street.
| A．very careful | B．too careful | C．carefully enough | D．enough careful |