Which One of the Statements Below Is True

Which one of the statements below is true?

A) When two atoms combine to form a chemical compound, they do so in many different proportions by mass.
B) When two different compounds combine to form an element, they do so in definite proportions by mass.
C) When two different elements combine to form a chemical compound, they do so in definite proportions by mass.
D) When two molecules combine in a chemical reaction, a number of different elements can be generated depending on the masses used.
E) When two different elements combine to form a mixture, they do so in definite proportions by atomic number.