what's the difference between 'puzzle' and 'doubt'?
Sure, the networked society allows us to gather information within a short time, but does it really speed up our ability to make better decisions? How do you balance the need for speed with sharp and correct thinking? That’s the ________on the minds of a lot of people these days, including Future Shock author, Alvin Toffler, who studies the idea in our cover story.
A. idea B. thought C. doubt D. puzzle

Why should choose 'puzzle' not 'doubt'?

Question

what's the difference between 'puzzle' and 'doubt'?
Sure, the networked society allows us to gather information within a short time, but does it really speed  up our ability to make better decisions? How do you balance the  need for speed with sharp and correct thinking? That’s the ________on the minds of a lot of people these days, including Future Shock author, Alvin Toffler, who studies the idea in our cover story. 
A. idea				B. thought			C. doubt				D. puzzle

Why should choose 'puzzle' not 'doubt'?

Answer

Let's look quickly at both words meanings. 
Puzzles are tricky problems that you have to figure out. 
Doubt when used as a noun means a feeling of uncertainty. The questions leading up to that empty blank don't support a feeling but a problem that needs to be solved. 
Place the word feeling instead of doubt into that blank. Is it just a feeling on their minds? No, it's a problem or a puzzle.