Which of the following is true regarding peer$-$to$-$peer $($P$2$P$)$ sharing:

a$.$ files can be permanently deleted or retrieved once another user on the P$2$P network has downloaded your files

b$.$ there is no need for concern if a P$2$P program asks you to disable or change the settings of your firewall

c$.$ closing the file$-$sharing program window $($clicking the x$)$ will always close your connection to the network

d$.$ some P$2$P programs open automatically every time you turn on your computer