story: Karen was taking a taxi to the store. When they arrived the driver realized he hadn't set the meter. They had no clue how much it cost. He waved Karen off, saying that it was on the house. Karen hesitated but then thanked him.
 selected sentence: They had no clue how much it cost.
The taxi driver has no idea how much the fare is >Causes/Enables> The taxi driver gives Karen the ride for free