Everyone knows that _______ is dangerous to play with fire, but _______ is difficult is to prevent children from playing with fire.

You look very beautiful in this dress and there is only one of this kind left here. I wonder    if you would like to buy ______. 

_________ it takes many years to gain the skills of an expert.

President Obama told television viewers that he saw ______ as his duty to run for a second term.

I wanted to buy a copy of The Youth, but there was     left in the bookshop．