There are 100 doors in a row, all doors are initially closed. A person walks through all doors multiple times and toggle (if open then close, if close then open) them in the following way: 

In the first walk, the person toggles every door 

In the second walk, the person toggles every second door, i.e., 2^nd, 4^th, 6^th, 8^th, … 

In the third walk, the person toggles every third door, i.e. 3^rd, 6^th, 9^th, … 

Likewise,

In the 100^th walk, the person toggles the 100^th door. 

Which doors are open in the end?