It was not until the beginning of the twentieth century________to explore space.
--I'm going downtown. --Come on. __________.
--Shall Mary come and play computer games? --No, __________ she has finished her homework.
--I'm sorry, I shouldn't have been so rude to you. --You___________ your temper but that's OK.
Whether ways will be found to stop pollution or not is just___________ worries the public.
I've visited a lot of different places and stayed in lots of different hotels, but none of them___________ this one.