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 $\frac{1}{4}$ ,and For the ship to be operational at least two of its engines must function. Let X denote the event that 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?

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