You may never be in an emergency situation. But if it happens, you should know how to get help. The telephone book in the United States has emergency numbers on the inside front cover. Look at the following table from the Boston telephone book. Notice that the numbers for the police and the fire department (部门) are the same. And it is an easy number to remember. If you are too unhappy or excited to remember any numbers at all, you can simply dial (拨打) “0” for an operator (接线员) in any emergency.

下列二次根式中，属于最简二次根式的是（       ）