QUESTION 6 What is the main disadvantage of user-level threads? They require a thread control block (TCB) They are not supported by all operating systems. OThey can block the whole process in case of a potentially blocking system call. They require system calls to create and terminate QUESTION 7 In a system that implements user-space threads, which two of the following operations invoke the process scheduler? (Check both.) mutex.wait0 Thread creation. A system call mutex.signal0 Master thread termination