a) Water is loss through TRANSPIRATION at the leaves and this pulls water up through the roots and the xylem tubes. Hence, water level decreases in the beaker as water is absorbed by the roots.

b)Oil is to prevent water loss from the beaker due to EVAPORATION. Note that oil is much less volatile than water.

c) VOLUME of water in the beaker at the beginning. Diameter and shape of beaker. Volume of oil that is put into both beakers.