GATE Papers >> CSE >> 2010 >> Question No 17

Let L1 be a recursive language. Let L2 and L3 be languages that are recursively enumerable but not recursive. Which of the following statements is not necessarily true?

• (A)  $\mathrm{L}2-\mathrm{L}1$ is recursively enumerable   
• (B)  $\mathrm{L}1-\mathrm{L}3$ is recursively enumerable   
• (C)  $\mathrm{L}2\cap \mathrm{L}3$ is recursively enumerable   
• (D)  $\mathrm{L}2\cup \mathrm{L}3$ is recursively enumerable