3. (3 points) In 2-3 sentences, explain why it is necessary for each thread to have its own stack. In 1 sentence, describe what would happen if threads shared a single stack? 4. (3 points) Whether you know it or not, you've seen and used a variation of signals extensively in your programming, particularly in Java. In 1 sentence explain what this variation on signals is, and in 2-3 sentences explain how the concepts we're discussing in terms of signals map on to that feature of Java