______ , he could not carry the rock to the other side of the river. 

 There are some health problems that, when ______ in time, can become bigger ones later on.

There is no doubt ______ you'll be warmly welcomed by people there.

海南三亚一中月考3，24）So difficult _______it to work out the problem that I decided to ask Tom for some advice.

 _______ by keeping down costs will Power Data hold its advantage over other companies.