Which of the following is the prefix for address 2000:0000:0000:0005:6000:0700:0080:0009, assuming a mask of /64?
a. 2000::5::/64
b. 2000::5:0:0:0:0/64
c. 2000:0:0:5::/64
d. 2000:0:0:5:0:0:0:0/64