A 20-g bullet moving at 1 000 m/s is fired through a one-kg block of wood emerging at a speed of 100 m/s. If the block had been originally at rest and is free to move, what is its resulting speed?
选项：

A:9 m/s;
B:18 m/s;
C:90 m/s;
D:900 m/s;
E:60 m/s;
F:600 m/s