You can put this solution on YOUR website! The key word here is metric which means that they're talking about millimeters, centimeters, meters, kilometers, etc...Inches are NOT part of the metric system (they're actually part of the English system)
So how long do you think the average key is? Just think of a meter stick and try to visualize the key right next to it. Would a millimeter be best used? A centimeter? A meter (ie the whole stick)? Or something else?