By selling two items at the same price, a person gains 20% on one item and loses 20% on the other. Then over all

Correct option is C

Concept:

Solution:
If a person gains x% on one item and loses x% on another item, the net result will always be a loss. 

Here, the common percentage is 20%, so the net loss is
Net loss=(20/10)²= 4%
Therefore, the person incurs a net loss of 4%.
Thus, the correct option is Option ( c).