What must happen before a chemical reaction can begin?

a the activation energy must be exceeded.
b the activation energy must be reached.
c the concentrations of products and reactants must be equal.
d the concentration of reactant molecules must be reduced.

If you fill your car tire to a pressure of 32 psi (pounds per square inch) on a hot summer day when the temperature is 35°c (95°f), what is the pressure (in psi) on a cold winter day when the temperature is -15°c (5°f)? assume no gas leaks out between measurements and the volume of the tire does not change.