Consider a linear function/operator f(x) . Which of the following statements are true?

A) f(x_1+x_2) = f(x_1) + f(x_2)
B) f(x_1 \cdot x_2) = f(x_1) \cdot f(x_2)
C) f(x_1) + f(x_2) = f(x_1+x_2)
D) f(x_1) \cdot f(x_2) = f(x_1 \cdot x_2)