What causes the piezoelectric effect?

ANS:B - pressure on a crystal

Piezo electric effect is the charge that accumulates in certain solid materials (notably crystals, certain ceramics, and biological matter such as bone, DNA and various proteins) in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure. It is derived from the Greek piezo or piezein which means to squeeze or press, and electric or electron.