$(20$ points$)$ The algorithm for summing n numbers using n processors in O$($logn$)$ time uses: $($a$)$ Concurrent read in that in one step more than one processors read a single memory cell. $($b$)$ No concurrent read is needed. $($c$)$ Concurrent write in that in one step more than one processors write a single memory cell. $($d$)$ No concurrent write is needed.