A computer's CPU can execute a program in $3$ minutes and $30$ seconds. In order to shorten the execution time, the program was analyzed and it turned out that part of the program can be executed in parallel. It is estimated that $70\%$ of the program can be done in parallel. $(9$ points$)$If we execute the program with $4$ identical CPUs in parallel, $($a$).$ estimate the speedup$($b$)$ estimate the time it will take to execute the same program, assuming all others remain unchanged. $($c$).$ estimate the limit to the speedup using $4$ identical CPUs.