Stephen Hawking believes that the earth is unlikely tobe the only planet _______ life has developed gradually

下列关于文学常识的说法中，正确的一项是（ ）

I am interested in travelling. And I have visited more than ten cities in China _______.

I can hardly work out the answer _____ thisdifficult question.

Unless ________ to speak,you should remain silent at the conference [ ]