Hermann Von Helmholtz, the great German psychophysicist, studied visual attention by observing his own abilities to process brief presentations (stimuli flashed for < 100 msec) of complex stimulus scenes. He observed that he could .

Ibelieve it's choice d.

If it's a smaller child or infant, place one on the middle of the chest and on the child's back. (if it is a bigger child with enough room for two, place one on the upper right/your left of the chest, and one on the lower left/your right) the pads should never touch each other. 

The answer is d because , safety almost always works, to prevent stds and pregnancy