Below given are some statements about electronic devices:

(A) Diodes can be used for rectifying an ac voltage.
(B) For semiconductors, band gap energy $E_g > 3 eV$.
(C) By changing the external applied voltage, junction barriers can be changed.
(D) p-n junction is the 'key' to all semiconductor devices.

Choose the correct answer from the options given below:

(A), (C) and (D) only

The correct answer is Option (4) → (A), (C) and (D) only