A ship is fitted with three engines $$E_1, E_2$$ and $$E_3$$. The engines function independently of each other with respective probabilities $$\frac{1}{2}, \frac{1}{4}$$ and $$\frac{1}{4}$$. For the ship to be operational at least two of its engines must function. LetX denote the eventthat the ship is operational and let $$X_1, X_2$$, and $$X_3$$ denote respectively the events that the engines $$E_1, E_2$$, and $$E_3$$ are functioning. Which of the following is (are) true ?