Even if most people do not rec0gnise anything when glancing at a picture, they can still make a right guess because the brain has done______.

A．It increased the students’ white blood cell.
B．It increased some students’ energy level.
C．It improved the students’ ability to play musical instruments.
D．It released a natural painkiller in some students’ bodies.

A．The Sources of strength.
B．Body building.
C．Strong heart.
D．Strength, fitness and endurance.