Underline the smallest length.  $10$ millimetres~ ~ $100$ centimetres~ ~ $10$ metres~ ~ $2$ kilometres~ ~ $2800$ centimetres~ ~ $2500$ metres~ ~ $100$ ~ millimetres~ ~ $100$ metres 
Overseas Competition->Knowledge Point->Calculation Modules->Comparing, Ordering and Estimating->Comparing and Ordering->Comparing and Ordering Quantities