The shortest wavelength in the Balmer series of hydrogen atom is equal to the shortest wavelength in the Brackett series of a hydrogen like atom of atomic number 'Z'. The value of 'Z' is

A radioactive sample has half-life of $5$ years. The percentage of fraction decayed in $10$ years will be

The material used for solar cell should have band gap
(A) equal to zero.
(B) equal to $0.08$ eV.
(C) between $1.0$ eV to $1.8$ eV.
(D) equal to $2$ eV.

For a two input logic gate, when the inputs are 'zero' and 'zero' the output is 'one'. When the inputs are 'one' and 'zero' the output is 'zero'. The type of logic gate is