Mark needs to lower the pitch of the guitar to play his songs. How can Mark lower the pitch of the guitar?
 A 
 Mark can make the strings tighter so they vibrate faster.
 B 
 Mark can make the strings looser so they vibrate faster.
 C 
 Mark can make the strings tighter so they vibrate slower.
 D 
 Mark can make the strings looser so they vibrate slower.