Harshit wanted to find out among the four locations - playground, science laboratory, school garden and the bus stop outside his school - which is the most polluted. He placed four glass tiles smeared with petroleum jelly at the four locations for the same period of time for one day and recorded his observation. However, his teacher told him that his experiment was not reliable because he did not ________.
AWet the glass tiles
BUse tiles made of different materials
CRepeat the experiment over a few days
DExamine the tiles with hand lens