In following question use ∈ or ∉ to indicate whether the given object is an element of the given set in the following problems.

a. 12 {1, 2, 3, 4, . . . }

b. 5 {x: x is a natural number greater than 5}

c. 6 {x: x is a natural number less than 6}

d. 3  ∅