Which of the following sets are null sets?

Question

$(i)\; A = { x : x \lt 1 \text{and} x \gt 3 } \quad (ii) B = { x : x^2 = 9 \text{ and} \; 3x = 7 }$

$(iii) C = { x : x^2 - 1 = 0, x \in R } \quad (iv) D = { x : x\; \text{is an even prime number} }$