the number of elements of a PRINCIPAL IDEAL DOMAIN can be

(1)15

(2)25

(3)35

(4)36

please give reason why?

2)which of ideal of the ring Z[i] of gaussian integer os Not maximal?

1)<i+i>

2)<1-i>

3)<2+i>

4)<3+i>

3)if Z(G) denote the center of a group G,then order of quotient group G/G(Z) cannot be

1) 4

2) 6

3) 15

4) 25

please give the reasons of above questions?i'm totally confused