Science 7 grade 7. can anyone me answer these science questions?
i need the answers soon as possible.

thermal expansion in a balloon occurs because

the gas cools off and causes the size of the balloon to change
when the gas heats up, the particles move closer together
when a gas is heated, the particles hit the walls of the container with greater force
when the gas cools, the particles move closer together

is a way thermal energy is transferred.
fill in the blank ?

a cellphone battery is what type of energy form?
a piston in an engine is what type of energy form?
the choices are :
nuclear, radiant ,sound, thermal, electric, mechnical