How Are DDR2 I/O Speeds Related to Clock Speeds

Multiple Choice

How are DDR2 I/O speeds related to clock speeds?

A) DDR2 I/O speeds are the same as the clock speeds.
B) DDR2 I/O speeds are double the clock speeds.
C) DDR2 I/O speeds are triple the clock speeds.
D) DDR2 I/O speeds are quadruple the clock speeds.