It is common problem  happen to all.

sometimes  answers provided by author can also be wrong.

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few tips

• when you are already in position to gain something, and by any effort you are adding in that gain, means it is enhance
• when you have no previous balance, means you start with 0 gain and making effort for gaining sothing , it will be exploit.
• when you are grabbing the opportunity as it is on the way is exploit. when you making it twice/thrice or any increase in amount, it is enhance.
• in enhance condition you will find that there is some comparation, word of comparativre degree will be used like better, more, double, increase etc. while in exploit condition , you are trying to unhide /explore the opportunity and grabbing it. 
• for example -A fisherman throws a fishing-net to catch fishes in a fishery pond and simply gets a good amount of fishes.
  

  Whereas the other fisherman spreads the net with Fishing bait in night and collects it in the morning and gets the double amount of fishes than the first one.

  The first fisherman exploits the opportunity.

  Second fisherman enhances the opportunity.