At the same temperature and pressure, balloons of equal volume always contain a. equal masses of gas.
b. equal numbers of molecules.
c. equal densities of gas.
d. equal numbers of atoms.

The net force is 100

You can look it up online and some images should pop up 

I'm pretty sure the answer is alcohol 

The internal energy of the system we can term it as total potential energy and kinetic energy. energy flows out of the surrounding from the system then we can say that the internal energy of the final state is lower than its initials. the sign of  δesystem is negative.δesystem = efinal - einitial=-ve  efinal  ∠einitial.