Piezo-electric crystals have many applications other than in ultrasound scanning. For example, they are used in:
a. Gas lighters (to produce a spark)
b. Inkjet printers (to break up the stream of ink into droplets)
c. Guitar pickups (to connect the guitar to an amplifier)
d. The auto-focus mechanism of some cameras (to move the lens back and forth).
For each of these examples, state whether the piezo-electric effect is being used to transfer energy in the vibrations of the crystal to electrical energy or the other way round.