In an electric circuit, two resistors with resistances x and y are connected in parallel. If R is the combined resistance of these two resistors, then the reciprocal of R is equal to the sum of the reciprocals of x and y. What is R if x is 4 Ohms and y is 5 Ohms? Select the correct option:
A) 7 / 9 B) 20 / 9 C) 7 / 20 D) 9 / 20 E) 11 / 20

Rationale: "1 / r = 1 / x + 1 / y 1 / r = 1 / 4 + 1 / 5 = 9 / 20 r = 20 / 9 the answer is b ."
Answer: B