Which of the following are empty sets? why : A = {a|a is a natural number smaller than zero}

Which of the following is an empty set ?

Which of the following are empty sets? why : B = {x|x^2=0}

Which of the following is the empty set ?

State with reason, which of the following sets ae empty : C = {a | a is a natural number less than zero.}

Which of the following are empty sets? why : C = {x|5x-2=0,x in N}

Which of the following has smaller size ?

Which of the following is in nature?