Which one of the following is a null vector

Which one of the following is a null set?

Which one of the following is a null set?

Which one of the following is a null set ?

Which one of the following is a null set ?

Which of the following is a null set?

If A={x:x is a multiple of 3} and B={x:x is a multiple of 4} and C={x:x is a multiple of 12}, then which one of the following is a null set?

Which of the following is a null set ?

Which one of the following is not a a vector product of two vectors ?

If A = {x:x is a multiple of 7}, B = {x:x is a multiple of 5} and C = {x:x is a multiple of 35}, then which one of the following is a null set ?