Answer:Reducing mass i.e. water
Explanation:
Frequency For given mass in glass is given by
[tex]f=\frac{1}{2\pi }\sqrt{\frac{k}{m}}[/tex]
where k =stiffness of the glass
m=mass of water in glass
from the above expression we can see that if mass is inversely Proportional to frequency
thus reducing mass we can increase frequency