---People often don’t appreciate what they have, do they? ---No, _______ they lose it.
| A．if | B．after | C．until | D．when |
Sunglasses can ___ us from strong sunlight.
| A．hold | B．prevernt | C．protect | D．make |
His father feels ___ his duty to help others.
| A．this | B．it | C．that | D．what |
I regret ___ harder in the past two years.
| A．not having studied | B．having studied | C．having not studied | D．not being studied |
Tom’t uncle must be an engineer, ____?
| A．mustn’t he | B．can't he | C．isn’t he | D．needn’t he |
I'm feeling blue, for they ate all the cake and _________left.
| A．none | B．everything | C．everyone | D．no one |