Answer:
"The runners passed over each hurdle in a flash."
The best replacement for hurdle in this sentence is barrier. Barrier is a obstacle that is often physical, so it makes the most sense.
"The largest hurdle in my plan to buy a car was lack of money."
The best way to replace hurdle in this sentence is to use problem. Lack of money is an unwelcome issue, so problem is best.
I hope this helps!
