If a user$-$level program X can modify the memory content of OS kernel, which of the following could happen? $($a$)$ X could terminate the execution of other user$-$level programs $($b$)$ X could modify the memory content of other programs $($c$)$ X could block the execution of interrupt handlers $($d$)$ X could modify the hard disk contents $($e$)$ X could crash the system user interface