When a posted speed is 70 mph , the minimum speed limit is what?

 In order to find your minimum speed, you would likely be subtracting 20 from the highest speed, therefore you minimum speed according to these guidelines would be 50 mph, so your sign would read as follows

Speed Limit
    75 mph
     Min.
   50 mph

The correct answer is a. when they started making tools that will them in cultivating lands, they could raise crops and that made them attached to one place for a longer period of time. after that, they started domesticating animals and building their own permanent settlements that resemble the ones we have today.