Asubstance composed of two or more elements in a fixed, definite proportion is
a. a homogeneous mixture.
b. a heterogeneous mixture.
c. a compound.
d. a solution.
e. an alloy.

Which of the following statements is true? a. most heat energy is easily recovered and used for useful actions. b. friction causes molecules to vibrate more slowly. burning air and gasoline in an c. engine changes chemical energy into mechanical energy. it is impossible to d. change mechanical energy into mechanical energy.